Would you (straight off the bat) trust a lady who cheated on her man with you.?

When we met she told me she had a boyfriend and I planned for us to just be friends, but she was on a date with me. She told me she wasn't happy in the relationship and he didn't show her value. He was acting like he was put off by her and watched porn instead of trying to be intimate with her. I felt sorry for her, she was so beautiful, so adorable and what I was hearing angered me. I decided to step in and be her man, show her love and care. Problem is I don't know if I should trust her as what she did was betrayal, when I was with her she talked to her man on the phone and it angered me. She lied to him and I sometimes think when we on the phone is she doing the same to me. I am runing our relationship by my mistrust and unfounded accusations. HELP!!!!