Should I do something?

My friend and I are both 17. She's always had trouble in relationships with guys fucking her over and things like that. So I guess to deal with what she's feeling she sleeps around quite a bit, which I have no problem with, she can do what she wants. And because she sleeps around, she gets attached very easily and rushes into relationships.
The problem now is that... The latest guy she's dating and has had sex with many times is 28 years old. Like are you kidding? I'm not trying to judge her but obviously her relationship is very very illegal. I told her I wouldn't do anything about it as long has she's safe and she said "he hasn't done anything yet." I don't know what to do, or whether or not to do anything. Any advice?