Your partner is going through the stages of grief.
He probably does not want you to say the other guy is a jerk. Because what does that make HIM? A fool for falling for a jerk? He's not at the stage where he can look at his ex with a realistic eye. His eyes are filled with tears right now.
So... if you are going to be doing the ministry of presence for your partner?
Ask how he feels. Ask if he hurts, and validate that it is NORMAL to feel yucky in break up times like this. Bring things that help him feel yucky better.
I don't know what he likes. But Kleenex is usually a good bet. What other things comfort your partner? Cup of tea? Silly movies? You would know.
Make safe emotional space for him to just talk and unload -- that he misses the ex, that he's mad at the ex, that he loves the ex. But be ONLY "Ear." It is not time to be "Ear with feedback" in the early stages.
Affirm YOU are still here for him. You cannot magically wave the yucky away, but you can endure it with him til it passes so he doesn't have to endure alone. Ask how you can best serve him right now -- just be Ear? Tell stories about totally unrelated to help take brain off hook? Where is his need today? This hour? (Knowing he's all over the map and it can change minute to minute even as he weathers his internal storms or emotion.)
Sooner or later he will get to the turning point. And if when he's there he wants to evaluate this relationship and THEN asks to get constructive feedback from you about whether the ex behaved well or not? Then you can give that sort of feedback.