You need to separate how you feel about him from his relationship with your daughter (that is what the court expects you to do). Unless he is drinking to excess or otherwise endangering her the court is not going to deny him overnight visitation based on the fact that you don't like him having a GF. My advice would be to let it go and see how this weekend trip works out, either it will not go so well and she won't ask to go again or she will have a good time.
I find that if I hand my ex enough rope he hangs himself everytime.