If the youngsters are the same age, is it still statuatory rape? Don't see how. On the other hand, if the boy was/is 18 and the girl 15/16 and it were discovered he would have to register as a sex offender. Which I also don't understand.

How well do you know the parents? If I knew them quite well, I might say something to them on the order of "Judging by what I saw I think your daughter and her boyfriend may be at the brink of something serious."

In other words, pretend you didn't see the actual activity.

But I don't know. I have never known, either, whether I would tell a good friend that her/his SO was cheating.