That is a tough one. You don't want your son's girlfriend to be left as charge of your son. Unfortunately, he can leave him with her while he has visitation with your son. Unless she does anything that endangers your son, you really can't do much.
Telling the girl to go home is not disorderly conduct. What did you say on her cell phone? My guess is that you got angry and said some things that your ex found offensive and was defending his girlfriend. That said, as much as you don't like it, you need to back off. If your ex is with her, she will be around your son. You want to have as good a relationship as you can with her for the sake of your son. You don't want her to take her anger towards you out on him.