My fiance best friend is a guy and he wants more than just friends with her?

My fiance tells me that he is her best friend they tex back and forth all the time and used to work together also. I recently found out that he wants more than just friends cus I seen a tex he sent her telling her he wants more. And when i confronted her about it she didnt admit he liker her at first then she later said he did. This guy is also a person i know and the last time we went out he was texing her and she was texing back and when i asked who she was texing she said someone else then later found out it was him. She said she lied to me cus she didnt want to loose his friend ship. What do I do. she says she doesnt like him but why would she lie to me about then? Also after I read that tex and asked her about it she now locks her phone she says i dont trust her is it me or is she hiding something??