My girlfriend was charged with assault after her ex attacked her at her house?

My girlfriend was at her house last night with her son when her ex husband showed up to pick up here son for the night. He told her that he has told her son that it is ok that he starts calling his new girlfriend mom. She reposnded back that she does not approve of it because she is her mom and no one else is. He got angry and rose his voice and said "You know I should set your straight sometimes for the things that you do!". His new girlfriend was standing right there and started to yell at my girlfriend. She asked them to leave because she didnt want any issues. They continued to yell and try and push their way into the house. She attempted to close the door but they pushed in and broke the screen door and pushed my girlfriend, she then pushed back. They ended up calling the police and told them that my girlfriend had attacked them by punching and ripping his shirt. He works in tree trimming and rippes his shirts everyday doing that. The cops believed what they said because she had no one that witnessed it. They had someone else drive over and take their son before the police showed up and they had him tell the police their story about her attacking them. She was charged with a class B misdemeanor for assault. She has no previous record of anything. He has a past of violence but sadly not recorded. What can she do? This is in the state of Utah by the way.