Tainted Beef

When you find out a girl has already slept with a close friend. (male or female, it's still beef)
See Tainted Sausage for a man.

Dude 1 'Bro I was gona sleep with Chloe, but then I found out you already had'
Dudette 1 'Ah man that sucks, doesn't matter though, she was tepid at best'
Dude 2 ' When a friend taints your prospect beef it hurts hard, ahh... Tainted Beef'