There is a crime called statutory rape. Per the legislative intent behind the creation of this criminal category, minors do not have legal capacity to be "willing".

The difference between rape and statutory rape is that rape is forced onto an unwilling adult victim, while statutory rape is not necessarily physically forced onto a minor, but the minor lacks the capacity to give consent.

I am not suggesting that you explain it to the friend, because right now he wouldn't, most likely, be receptive to this kind of explanation, but for your own purposes, you should know that. Your post demonstrates that you do not.