This time last year, my best friend of around 10 years (B) started dating a new coworker (C) of ours. From an outside perspective, the relationship seemed somewhat broken --- My friend B was really into him, and even though C seemed to be really into her, he never seemed like he wanted to commit to anything serious. Over the course of about six months they broke up once or twice, until eventually C ended it, because he just didn't think B was 'the one'.
About two months ago, two things happened. The first, a mutual friend of ours (D) went out for a drink with C, which caused the two of them to stop being friends. I asked D what had happened, and she said "C told me some things that made him seem really manipulative, like he was just using B for something". But she refused to tell me exactly what was said.
Secondly, I went out for a drink with C, who I was still vaguely friendly with, and he asked me out of the blue if I would consider a relationship with him. I told him no, the drinks ended shortly after, and I never mentioned it to anybody. Maybe this is me jumping to conclusions, but the way I understood things after that point, C was never fully committed to B because he was after me instead (we both met C at the same time), and this was the essence of what he had said to D.
I was hoping that I could just pretend none of this had ever happened and forget about C altogether, until two weeks ago, B told me that C had suggested they should get back together. I told B that it was a terrible idea, that she should just move on from him. I told her that C had already made his commitment to her clear in the first six months of their relationship, and that she deserved somebody who actually loved her.
B ignored my advice and decided to get back together with C.
How can I protect my best friend from being hurt any more in this situation? I feel as though she is walking back into a relationship without knowing some damning and important things about the man she loves, but at the same time, I feel that telling her those things would do even more damage.