GuyRule #601.0: She's My Boy's Ex, But . . .
It's only okay to get on another guy's ex-girlfriend if the following rules apply:
a) she's damn fine
b) he's not either your brother or one of your 3 best friends
c) you're drunk
d) you know that they aren't going to be back together next week.
Or if this one rule applies it's okay no matter what:
e) he stole her from you first. -RvrChsr
Already said it's a no no. -WP