When a good friend of mine was in a relationship that I really, really didn't like (bad dynamics, horrible behaviour, past cheating, she acknowledged his MANY faults but believed he was trying to change - I didn't!), I told her straight out that I didn't like him and didn't want him at my house, but I respected her wish to give him another chance. I wasn't going to nag her about him and if he did something she needed to vent about I'd be a sympathetic ear without being nasty or "I told you so" about it. I'd be polite to and about him, and I wouldn't criticise him unless she asked me for my unvarnished opinion of something he did, but if she asked she was getting it warts and all.
Things went south. He's no longer allowed anywhere near her, and if he ever shows up here I'm calling the cops. We're still friends... and if I'd given in to the temptation to badmouth him to her at every opportunity (and trust me, there were a LOT of opportunities!) I don't think that would be true any more. Criticising someone's SO too often comes across as criticising their taste in SOs, which becomes criticising them. "Dear Deity, he's awful and he treats you badly! Why do you still love him?!" becomes "Only an idiot would love that guy, therefore you're an idiot," in their head.
If you can do it gently and you don't think she'll react too badly, I'd let her know you're still a bit unhappy about his behaviour. Make it about things he does, not things he is - "I get really uncomfortable when he criticises you in public, it makes him sound mean," instead of "He's mean to you." Then stop. Be her friend, to the extent that you want to be her friend. Don't try to deepen the relationship past what you're comfortable with to make it easier to 'save' her - you can't. If she really needs saving, she's going to have to save herself, and that'll only happen when she's a) convinced and b) ready. If you push, she'll back away from you in defence of her shiny happy relationship, and then she won't feel she can come to you for support if/when things go badly.