how do I help her here?

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.