Relationship Advice. I need help.?

[MM]

Tell her you wish you'd spoken up sooner, because you're not entirely surprised, even though you are upset and disappointed. But wishing she could have you both or that someone doesn't have to get hurt isn't possible, and she's going to have to make a choice. If she does want to fix things with you, then she needs to break off all contact with him outside of what's absolutely necessary for her job, and work with you to figure out how this happened and what you weren't providing her that he was so that it doesn't happen again. If she's not willing to do that, then unfortunately, as much as you don't want to let her go, you're going to have to.