I appreciate all your opinions.
What I've been doing thus far is kind of a combination of many of the suggestions given to me here. I'm supportive of him, at the moment. When he brings Cassie up, I usually say something like, "I can't really speak on that." I've already told him that because I don't know everything that's gone on in her life, I'm not qualified to really judge her.
However! When he asks, I will give my opinion, although I would never directly tell him to dump her. I generally tell him that my big concern is seeing him being used and hurt.
Yes, I think he's beginning to see that he needs to do SOMETHING, but he isn't sure what, yet. And yes, I do believe he needs to feel needed, and he feeds a bit on the drama.
As for Cassie, I've known her far longer than I've known Mike. I met Mike when Cassie took up with him, and spent a few years just knowing him as the guy she was seeing on the side. Even though she's never been anything but nice to me, I've never been comfortable with her. This is an embarrassingly subjective thing to say, but she's just one of those people who switches on my "be wary" button. She always has been.
So thank you all for the advice. For now, I think all I can do is keep on keeping on the way I have been, and letting it play out, hoping that however it ends up, everybody is okay with it.