Sounds like a rather perplexing predicament
I'm afraid I can't be of much help. I don't have much experience with this sort of thing. But I can say this: once the guy turns 18, it's illegal for him to date your friend until she's also 18. But even then, the age gap is way too big to ignore. If you want my opinion, what this guy is doing is morally wrong (and also pretty illegal). This is a matter for those who realize this fact.
The way I see it, this has two major outcomes:
1. The guy realizes what he's doing is wrong (and illegal) and quits while he's ahead
2. The guy doesn't realize what he's doing is wrong (and illegal) and suffers the consequences
I believe that the latter is more likely.
That's all I can say. I don't think it's too helpful application-wise, but it might provide some sort of foresight. Good luck with this.
