It's a little tough to give good advice on so little info, but since these two aren't married (and I'm assuming no kids involved), I think she should tell him she needs to talk to him, and to request a meeting in a public place. When he arrives, she should look as gorgeous as possible, muster up all her courage (she should role-play this ahead of time), look him in the eye, and THANK HIM.
Tell him that "I just want to thank you, because I had been having a difficult time deciding on whether or not you were someone who I could spend a future with, and remain committed to. But now that you've lied to me about there being someone else -- and please, don't try to deny it, because we both know it's true -- it's made me realize that I don't want to remain in a relationship with someone who cares so little about me, and who shows such poor character. I wish you well, I really do, and thanks for some really fun times."
And then I think she should get up, and walk out, and go pitch black.
