I am not sure there is anything she can do. In that situation she could have waited until the call was over and spoke to him about it. In other cases, a person needs to weigh out whether saying anything in a given moment is necessary. I suspect there is more going on with the girlfriend. Either she is very immature or there are other issues she is dealing with emotionally. How old is your son, and does he live at home? I would add that if he was on a trip with his mother to spend time with her then that should have been his focus. But, you can't dictate that either. These things are rarely easy.