When ever I Wish to talk about it she say I am dwelling on the past which was only a month ago and she claims not to know why or can't remember what they talked about. I think it was more than what she is telling me and I don't think she is being completely honest with me. What can I say to her or how to talk to her without starting a fight and creating a hostile environment.

Drop it and watch for more 'red flags'
I learned the more you nag about something the more a cheating spouse will go out of their way to hide things.
If you leave it alone they get sloppy.
Maybe it was innocent and should be left in the past.

Say you don't mind her talking to him if you honestly don't, and just try and drop it into conversations about work. If she doesn't sa anything is happening trust her. If you tell her she is lying she won't like it one little bit