That's Up to his current instructor.

Yeah. This. Whenever a color belt or striped belt appears at our gym from another gym, especially out of the Houston area, we use our magical black belt powers to analyze and figure out where they are and how we will advance this student. But I have said too much...

This is probably the best answer you're gonna get. Best thing this dude can do is get somebody who has his back, now that Lasu is presumably out of the picture for him. I'd say it shouldn't be a problem in the long run, and it's way easier than if he was a black belt.

Speaking of which, I was in the position of voting for a black belt on here whose promoting instructor probably would not make the cut against the fearsome team of downvoters here on beltchecker. In the end, I wound up giving him my massive +50, since this individual in question has since been taken under the wing of a larger organization, all of the time in grade is there, and he's fairly well-known and respected as a black belt in this region.

If his new instructor recognizes his purple and vouches for him then he has nothing to worry about. His conscience should be clear. He acted in good faith while Lasu did not.
He knowingly defrauded his students.
The student is not to blame.
Good for him for switching schools.

My belief is if he explains the situation in his lower belts in comments of the belt history, and his new coach vouches for him. He is ok. He is acting in good faith.

One thing to add, is that dudeman needs to be honest to his current instructor about the previous situation.

Sometimes, people are so embarrassed or pissed off about being conned, they can come up with a bit of revisionist history.

Seen it happen. It's understandable. But don't do that.

I don't think he's got anything to worry about. Either his current instructor reckons he is at the level he should be, or he's below par for his current belt and he has to wait longer for the next one. That's what I would do (my instructor promotes my students higher than blue, but so far he's agreed with the people I've put forward). :)

Generally if someone shows up at our gym with a rank that does not match their skill level, then usually they just sit with that blue belt or purple belt until they're skill level matches that rank. It may take some time. I have seen it a couple of guys over the years self-demote some stripes off of their belt. I think it hurts the ego if a two-stripe purple belt is getting handled by a new blue Belt regularly. Particularly if their age and weight is comparable. But, if they have a reasonable backstory as to who promoted them and if the timeline is reasonable, they can wear the rank. After all, the color ribbon that holds your uniform together honestly means very little. We all know that.

I don't see harm in him holding his purple belt as he achieves that level. And if his instructor now feels he is a purple belt great! Just keep grinding

I'm not a black belt, and in my gym brown belts aren't the ones doing any promotions. However I agree with what has already been shared, and feel like I experienced a bit of this years ago when I was a blue belt. I don't want to give the impression that I was "conned" or taught by someone unqualified, but when I moved to a new area as a newly promoted blue belt, I regularly got tapped by white belts with a couple stripes. I didn't feel like I was up to par with the caliber that school operated at and offered to turn my belt in and start from scratch. The instructor quickly told me to train hard until I earn it, but he wasn't going to disrespect my former instructor by taking it away. If anything, it motivated me to train more often to get up to par. The day I received my first stripe on my blue belt at that school, it was like getting promoted all over again. That was an eye-opening experience and I will never forget it.

I have had this.. had guys come in with a purple and worked with them and wondered who the hell gave them a blue.. Had a guy at almost 300 lbs and verified his coach had given him a purple lose in under a minute to 3 athletic whit belts around 250.. The next time he came in a 6'4" 200 lb 17 year old put on a submission clinic on him..

What I do is pull them aside and talk to them as someone did give them a belt.. I explain the level I think they are at.. Tell them we are in a combat sport and the proof is in the pudding not around the belly.. I give them the option of keeping the belt or going where I think they belong but I tell them that if they keep the belt they can NOT compete as part of my team and honestly to expect the lower belts to come hard because in combat sports there is a food chain and those lower are always trying to climb and break the weak..

What they do tells me a lot about the person

Interesting: how many of them went for a demotion, if you can give a rough estimate?

This is an interesting question, and it IMO opinion breaks down into a few schools of thoughts. Ultimately it is up to the new professor what belt a person is, and asses the situation accordingly. If someone comes in with a belt ranking from another school, I'll honor that ranking as is, and then see where they are at in regards knowledge and technique, as well as how they spar. I know of guys who earned a ranking, and maybe throughout the course of life, or time away and age combined, maybe they lost a step or three, maybe their timing isn't as sharp, maybe they walked away for a while from BJJ.... So their best foot may not be forward when training starts, but they can work back into their form with repetition and practice. I have seen older, taken time away purple and brown belts, and even a couple of black belts, struggle and even lose to competition young blue belts (who are bigger and stronger), it happens some times. Look with all due respect, I'm sure that some of you lower ranked belts, and definitely some black belts could rag doll a lot of the living red belts and coral belts, because they are OLD now (I'd BODY up Helio and Carlos if they were still alive LOL). All that said, if a person comes in and is just lost as to what the hell is going on, doesn't know basics or defenses, and just looks lost in every position, and they claim to be a high level belt.... THEN I would question the validity of the belt.

Now the other thought I have is, it's up to the practitioner to truly look into their own training and ranking. We famously saw Enson Inoue a few years back, after taking time away from BJJ for over a decade, go back and demote himself back to Purple Belt from his Black Belt, because he felt like there was so much to learn, and he was learning things for the first time. If you as a practitioner walk into a place, and you are getting handled by lower belts (not for the aforementioned time away issue, or age/life/injury vs new young steroid beast issue, but for being legitimately lost in what to do), maybe talk to your new professor and say "maybe I'm not what this belt represents".

I also have seen some schools make competitors of every belt level (even black) start back as white belts, that I feel is a bit ridiculous, especially if they are active in the art/sport, all so that the instructor can claim that student as their own belt..... and sandbag guys in tournaments to make their gym look better..... I'm not naming names, but it does seem to be a thing that one well known gym and their affiliates do to my direct knowledge and experience with them.... a school that seems to have a lot of athletes from All The Other Schools.

Anyway, that's my take, ultimately it's on the new coach, and at the end of the day, it's also on the practitioner to be honest about their skillset.

I know someone who went to a new school as a new Black Belt and after getting dominated by the new schools brown belts ask the teacher if it was okay to go back to Brown because he wanted to receive his Black from the new teacher because he no longer felt he was a Black.. This is the attitude of a martial artist

This very thing happened to Rodrigo Gracie when he moved over to Renzo's, if memory serves.

I allow anyone that comes in to train to wear whatever color belt they earned elsewhere. I don't give them the third degree. If you want to come in with a brown belt and train, no problems from me. I just make it clear that you will be treated as the belt you wear. If you get tuned up a little, that's on you, not on me.