I normally don't talk crap about the exes of friends cause they are often my friends as well. My opinion of someone doesn't change at the moment of break-up. If the now-ex didn't strike me wrong before hand he/she won't now. Of course, if they did bother me before, I probably would have told my friend if I cared enough to bother. If I do have a problem with the before-ex, out of deference to my friend's choice, I will tend to keep it toned down or only tell the friend once. After that they are on their own.
I do make a point though to ask my really good friends, after one of their break-ups, if the ex is persona non grata (which can happen for any number of reasons). I won't go after them to rip them a new one, but if my friend (and it has to be a good friend) gives that nod, I will refrain for including the ex in my social circle. I do this mostly because if they are uncomfortable, I don't want to put them in close proximity.
Now, did I keep all the pronouns straight?