Should I tell my friend that she's moving a little fast with her boyfriend?

okay, to give a little background info my friend, Milly, is not a virgin. She was 14 when she had with 2 guys & about a week ago she turned 15. That may have seemed unnecessary but hear me out completely. She's been dating this guy for about 4.5 months now. He's not a good boyfriend, like he lies to her and she knows it and they're not even supposed to be dating, since her boyfriend Adam's mom made them breakup but now they sneak around to see each other. I believe that since her boyfriend lives by the mall she'll take me as an excuse for going bit then she'll meet up with Adam. Okay these next few details will be a bit mature. Saturday, the last time we went to the mall (me, Milly, and 2 other friends) we all decided to go outside for a little bit. I went inside to use the bathroom but as I was coming back I saw the 2 other people inside for some reason. They said it was because Milly & Adam started taking each others clothes off and started having ! I asked Milly about it, she said he only fingered her. I just don't wanna see her make a mistake and screw her life up. I'm not the person to get involved in peoples' relationships, but I think she should wait awhile. Should I even tell her what I'm feeling? Is it a bad idea? How could I get her to listen? I hope all of that made sense, I'm just a concerned friend.