Over time I became friends with a colleague who is the assistant of my manager. We are not in the same team but we have to communicate at work. I have noticed that when we are outside of work (ex: eating dinner together etc.) or we are alone, she is nice to me. But when we need to talk at work or in a group, she contradicts me, tries to prove me wrong, and makes personal comments (about weight or age, for example). I think this affects my manager's personal opinions about me.
I tried to talk to her about it, but she denied it. I don't want to be friends with this person anymore because of her behavior, and I don't want to bring this situation up at the workplace because it is personal.
What steps can a person take to shift an unhealthy friendly relationship at work back to a professional relationship?