Need help!!

My best friend (yes, I really mean my best friend) is in a relationship and things are starting to get sticky. They were together once before. Her BF cheated on her during their last relationship. She doesn't know about this. I found out recently that he just cheated on her this time, too. Only once. But he feels sincerely bad about it. What can I do? I don't want to tell my friend, but I know she's already made future plans for their relationship.