What to do if you know your girl is cheating on you?

i have been seeing someone for the past 9 months. When we started off, I told her that if for any reason she wanted to be with someone else on top of me, then so be it. I just wanted to know that they existed and who they were.

It doesnt bother me that she sees someone else as I am really not a jealous person... the problem is that she has been telling me that she has been seeing a female friend of hers when that same friend says that they hadn't spoken in a few weeks. Then, the friend said that she knee that my girlfriend was hanging out with another guy for the AST few weeks.

Also, I was friends with the other friend well before the two of us started dating.

So, at a bare minimum, she is lying to me, and that is the only thing that I told her that I would not stand for.

What should I do?