I have to agree with Sarahsweets here. My first thought was, "You don't allow someone to cheat." Everyone has free will to do what they want and her boyfriend is choosing to cheat. He could have ended the relationship or talk things out but he chooses to cheat. This is not your friend's fault. Even if a partner is treating another one poorly or being hot and cold, that's no excuse for cheating. Put the blame where it belongs: on your friend's boyfriend.

However, that is their business. I've learned that it's not too helpful to get involved in friends' romantic relationships. It gets very messy. Unless the relationship is downright horrible or abusive, then I'll get involved. But this just sounds like two people who need to communicate but are both choosing to run away from the problem.

I would focus more on your friendship with her and what you want to do regarding it. Do you want to set boundaries? Do you want to distance yourself from her? Do you want to communicate?

I wish you all the best.