Is this considered cheating?

My girlfriend and i live 2 hours apart. She has a best friend (male) who she has vacationed with prior to our relationship. She claims that they have slept together but nothing has ever happened. My girlfriend told me that he and his girlfriend were going to take her and her two kids (ages 7 & 8) to the county fair for the evening. I was fine w it bc she assured me that he was bringing his girlfriend. He picked her up at 5 ( the fair is a 15 min drive). I asked her to call when she got home. At 10:30pm I called her cell....no answer....11, 11:30, 12...no answer. The fair closes @ midnight. She called me at 12:40am to let me know that she got home. When I pushed the issue she admitted that his girlfriend was in NYC. She claimed that the girlfriend decided to go at the last min & she had no idea until he arrived @ her house.