Most Helpful Girl
I have in my naive teenage days gone back to a guy who cheated on me several times, but I will never again. Like already said, once a cheater always a cheater. If he was willing to disrespect her and the relationship then, I doubt he's magically changed now.
There's no excuse for not having some self control and working on any issues in the relationship or breaking up if you want to be with other people. If her being away was too much for him to handle he should of been an adult and communicated that its not going to work, not give her false hope and betray her.
However love is not rational so your friend will obviously do what she wants and all you can do is give her advice (being too pushy will just push her away) and be there for her for whatever happens next... but I'm betting it won't end well if she goes back. Eventually she'll have enough and learn from it and decide to find someone who's going to treat her right.