Well, I think technically it should have had a 4473 done on it, because in Colorado you can only do a personal transfer between immediate family without a BGC. Immediate family is blood related, extending to uncles aunts etc. No inlaws.
If it were me? I wouldn't care one bit. As long as he's not a prohibited person, shrug. I would also have this thread nuked.
Here you go for reference:
https://www.cashmyguns.com/blog/how-to-sell-a-gun-in-colorado/At worst, it looks like a misdemeanor if found out. Those last 3 words are the key.
Eta - it's against written law. I guess that's an ethical call for you. Maybe advise him about it, and tell him to stfu about where he got it, or tell him to exchange back and do it the "legal" way. Up to you I suppose.
Like I said, if I'm in that same position as you, as long as he's an otherwise good kid and i know he isn't prohibited...shrug. I'm not involved in their deal so I don't really care. It can't come back to me. In your position.
This is not legal advise. I am not a lawyer.