Is it ok for my girlfriend to go out with another guy to the movies?

We had plans to go see Bruno on a sunday because she did not want to go home late from work on a friday. But little did i know a mutual friend from college saw her after work and asked her to the movies and she eccepted. Guess what movie they saw? Bruno. Fuked up huh? then she gets home 3:00 am in the morning. Im pissed, and she thinks its ok but if i did that she said she would be mad.