First of all, I agree that most likely nothing will come of this. It's just a matter of time and him coming across girls at school that he could date.
Honestly, I would tell son, exactly what you said here. That if her parents don't approve of her communicating with him, that they definitely won't be happy about you getting involved. Tell him that you'll help "him" in any way possible in dealing with her issues.
I mean, you can't stop it, but you certainly don't want to promote it either.
