Don't let her cheat with him. Cuz she will wanna go back to him cuz she hurt his feelings and will leave you back. It wouldn't be nice to hurt someone and then hurt you as well.. But if you really like you then you just have to talk to her... and if she decides that she doesnt want to stay with him then there you go... but like i said if she goes back she'll forget about you and you never know if he rejects and shes all alone... i would not like to get anyones feelings hurt if i was you:) So maybe talk to her see how it goes and if not just find someone you really like:) Hope this helps:)
Don't let her cheat. She'll regret it when he finds out and he's hurt and you can never build a relationship if it's based on one of you cheating. Just tell her how you feel and if she likes you enough she'll let him go. If she stays with him than forget about her, she's not worth it.