If the Bitcoin received was mixed before payment, legally nothing can (more like should with how shitty governments are) be done to either party.

That being said, utilizing Bitcoin is enough to put you under governmental scrunity anyway. So tell them to get fucked if they come at your door.

I'd tell your friend to think about it the same way as someone paying with dirty cash: they don't know it was earned through less than legitmate means, so they're not held liable for accepting it so long as they assumed it was earned legitimately.