[Long Post] My ex seems to be in a toxic relationship and I don't know how to help

My ex and I (I'll call him B) recently started to talk again on the phone since he changed schools. Originally I contacted him because I was unsure of where we stood and whether or not there was still friendship there. We cleared the air about everything and now it's like it never happened. 

Now B and I are really close friends, so he tells me pretty much everything about his relationship. At first, he would tell me how much he loves his new girlfriend (who I will refer to as A) and how he wants to marry her one day and all that jazz. I was happy that he had moved on and found someone to care about. However one day, B called me in tears because of some things that A had said. This was a couple months ago, so I don't exactly remember what it was, but B ended up spilling a lot of things about his relationship: how A hangs out with her toxic ex (and my toxic ex/former best friend of B. Can be referred to as C.), flirting with him and running to him for comfort instead of B. A will bring up C to B during inappropriate times, like when she and B are being "intimate". B confessed to me that he wouldn't be surprised if A cheated on him with C. A will do something to B and at the end of it all, she will have manipulated the situation to make B end up apologizing for even being upset. A has even gotten upset that B was talking to me on the phone and almost made him stop being friends with me.

As time has gone on and I've talked more to B about his relationship, the less it's "I love her so much" and the more it's "she's such an asshole sometimes". B has no trust in A, and since he goes to a different school, he can never be sure that A and C aren't doing anything behind his back. One thing that will stick with me from one of our conversations is when he said that he doesn't even know what he's doing anymore or who he's doing it for. He doesn't even feel like himself.

I've tried to give him every piece of advice I can: to stop letting her run over him, to communicate with her about boundaries, to just break up with her because she sucks. He says that if his friend were in a relationship like his, he would tell them to break up, and that he doesn't know WHY he won't just break up with A. I asked what it would take her doing for him to call things off, and he said cheating on him. B and I both know she's been dancing on that line for a fine minute, and it's only a matter of time before she crosses it.

I don't want B to get hurt. I care about him a lot, and now there's also the issue of my feelings getting in the way of my judgement because late night conversations between exes tend to reveal a lot of emotions. I just want him to be happy, and this girl is breaking him down to the point where he's not the same B I know. Is there any advice I can offer him? What do I do if A hurts him? Is there a way I can get him to realize that his relationship is not a good one and get out of it?