You are really in no position to tell her who she can't see.
I posted this elsewhere, and may have already told you about it. I had something similar. My W has a friend who I think is a bad influence. My W told me about a couple of their conversations, and I realized that, at least in my case, my W was making her OWN decisions.
Her friend, like most friends, will encourage her to do what SHE (your W) wants to do. If your W wants a divorce, her friend will emotionally affirm her. If your W wants to stay married, her friend will emotionally affirm her.
Your W will have her own set of boundaries with her friends. There is another thread on this board titled started by Bummedout. If you haven't read it, please do. It will give you great input by a lot of posters on this board about this very subject.