best friend and her boyfriend

My best friend of 10 almost 11 years is going through a rough time. Her boyfriend has recently broken up with her, they're still friends and hangout, I guess it's just not the same. They've been together almost a year, now long story short, she cheated on him 3 times with her ex, like had sex 3 times, not once or twice, but 3 times! However, this isn't why he broke up with her, he actually isn't aware of the situation. She promises to never do it again, but she wants me to talk to her boyfriend/ex.. not sure.. about their relationship, she wants me to try to convince him that maybe they should stay together, now, I don't feel like it's my place to tell him that she cheated, I wouldn't do that, but I also feel like I shouldn't try to help her relationship, bc cheating is something I don't condone at all! I love my best friend, but just because we are super close does not mean i feel like it's right, I just need some advice on what to do, or how to approach this please?