She's screwing you over, and you wanna let her off nicely? I'd chew her up and spit her out.
Stop talking to her is one solution, telling her you don't want to be a part of the relationship anymore is an other. Look, she already knows your interested, there's absolutely no reason to tell her to pick you or him, just do it.
Seriously, what is she going to say when you say that phrase? "Oh my god you're the meanest guy ever! I'm never talking to you again!"? I doubt it, and even if she does, take it, because she's not worth your trouble.