How do i trust her again??

ok so i had/have this friend named alex...and one time when me and my fiancee were arguing and i left wit my daughter and went to my dads house....well he told me that he was helping a friend move and i didnt talk to him for like awhile...and then he even said we werent together for the weekend cuz he needed to figure some stuff out cuz i left at a time when he needed me...well he ended up leaveing wit my friend alex and her friend brent...well alex has told me b4 that she thinks dougs hott (fiancee) and well yea they ended up goin to a party and drank and ****! and i calld her that night and asked what was goin on and she said nothing and i had told her what was goin on and **** and she didnt even mention him! and the next moring i found out that thats where he was and i calld her fone and she hung up on me and ****! and were suposd to be friend and **** but i cant trust her!!!! shes a really nice person i guess....what do i do??