Divorce is horrible. It is the pain that keeps giving. If you have a friend going through it try and be as comforting as you can. There is no need to try and make her/him feel better by trashing the soon to be ex. Doing that doesn't make the wounded feel better, if anything they feel worse because they begin to wonder how could they've gone so wrong and then they're off and running to self-flagellation.
Unless you're on the witness stand, this is not the time to bring out dirty, little secrets you've harbored for years about your friend's mate. Never, never reveal rumors because that's probably all they are.
There is never a need to say, "I never saw what you saw in him anyway." You weren't supposed to. "I never liked him." You didn't have to like him. "I didn't like the way he treated you." This may be a fact for some but now that the marriage is over there's no reason for you to talk about what you liked and didn't like now.
Now is the time for all good friends to show the good. Do the listening and keep your mouth shut. The divorce tunnel is long and dark, some how try and help your friend see the light in the future without dragging in the past. Think of what would make you feel good to hear if you were in the same situation. Calling your friend's ex an asshole is beyond terrible especially if they have children because then you're calling the children part assholes. You're also calling your friend a bigger one. This is when we encourage talking behind one's back. It is far less hurtful and harmful.
Divorce is loss. When people die others always remember them as heroes, as the kindest, nicest, would do anything for you kind of person. Maybe your friend's mate wasn't all of that but once upon of time they were in her eyes. Keep that in mind when she reaches out for you. Impress upon them how good they are and how good the relationship was once and now it's not. Like Miles Davis said about one of his marriages, "It was good and then it wasn't good anymore." This may not be music to everyone's ears but it's true.