I think it sort of depends how the breakup was. If you just didn't get along and mutually broke up (or in many cases it SHOULD have been mutual, only one person is too blind to see it at the time), then it's nobody's fault and no reason to act all weird. If the X was cheating, abusive, or otherwise a bastard, than friends have every right to hate him (or her), and really, the bastard deserves whatever shit he (or she) gets.