I'll jump on the "stay the hell out of it" bandwagon. Sounds like your mom has made up her mind and isn't coming to you for advice, so she's not likely to appreciate it (or take it) if you give it to her. It's completely reasonable to tell her you're sad about them breaking up, you still feel close to your step-dad and plan to stay in touch with him, and that you're not interested in hearing her speak badly about him. And maybe add that while you accept that they both have the right to pursue happiness as they best see fit, you don't want to be put in the middle of their breakup and would prefer to spend your time together talking about other things.
As for deciding who is right or wrong in the breakup, that's probably not worth a ton of your time either. It's pretty hard to read relationships from the outside. And to quote Uncle Jack from Breaking Bad, "the heart wants what the heart wants, right?" She'll have to live out the consequences of her decisions, same as the rest of us. But assuming you still want to have a connection with her going forward, you're probably best off staying out of it.