interesting discussion. I think it depends on the circumstance. It's good to have trust.. but at the same time, the only thing I mostly trust is gut instincts.
I find its useful to consider how you got involved with the person in question. People are creatures of habit.. so if you got in quick, easy, as a casual friend, or by accident.. then probably best to keep it movin. Not to say there cant be exceptions to that, but generally speaking.
I probably wouldn't get involved in the first place with a lady who kept some guy best friends.