Good Friend, Turned Bad.

So I have this friend. Janell And I use to date this guy, Marcus. I broke up with him 6 years ago. It was a long painful relationship that I would never want to go down that road again. He cheated on me, i cheated on him. Well I met him out for drinks one night, we still enjoy each others company at times. I took what i thought was my best friend with me, well apparently they hit it off and she ended up meeting up with him later and they slept together. I told her she was wrong to do that and you don't hook up with your friends ex boyfriend. She doesn't understand why I am so pissed????