how do I keep her from doing something she'll regret?

Question

So I turned 15 in January and I'm in my freshman year of HS.
Anyway my best friend turns 15 in June.
She's been dating her boyfriend who's a sophomore for a year in April. 
He's older but up until now he's done nothing but respect her boundaries...
Last Friday he decided he needed break. But it wasn't bc he needed time it was bc wanted to go fuck around with another girl.
My best friend had her dad leave in 3rd grade and she's still got a lot of issues with guys leaving her, and he's the only guy she trusts in her life.
She got desperate to get him back and promised to go all the way with him, if he took her back. 
I don't know what to say to her. I know she's not ready for that, and I know their relationship definitely isn't. I doubt he even is. I don't know what to tell her.
He's cheated once in the past (just a kiss I think) and she got over it. I just don't want her doing anything she'll regret and I think that she shouldn't feel like she has to. Sex is a big step and something they both need to be ready for first. 
All of you ladies are older, and anything you have to say is appreciated. I don't know what to say to her and I don't want to go about this the wrong way. 
I just want to do the right thing here. Thank you all for your advice.

Answer

I would help her realize that any man that will only stay with her if she will go all the way with him is not deserving of her. Just keep giving her support and love. Let her know honestly how you feel about the situation

Answer

This really isn't your problem to solve nor can you prevent her from getting hurt. Tell her that you don't think it's a good idea but this is her life. She's going to screw up and if you get to involved then it could affect the friendship but again this isn't for you to fix. She has issues that only she can fix by seeking help. But mistakes happen. Ask yourself if you can still be there if she doesn't do what you want her to do.

Answer

Alexis has a point. Try to tell her that any man (boy) that will only be with you because you sleep with them is not worth it. But you know to know that it can maen that you less her as a friend. Remember she is going to do what she want no matter what you tell her. I had a friend in high school that was seeing a 22 year old because he was cute. She got pregnant and they get married but now over 10 years later they are not together because he cheated on her. She was 16 when she got pregnant. I still can't believe that her parents did not Press charges on him. I had told her I don't know how many times that he was no good. I lost her as a friend.

how do I keep her from doing something she'll regret?

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