If you ask him to choose between his mother and his girlfriend, he will choose the person he lives with. She is the most important person in his life now.

As I said, you don't have to like her. Maybe, if I met her, I mightn't like her either. Sometimes the best we can do is put up with someone. If she is flat out rude or disrespectful to you, then you have a right to object. But your objection has to be to a specific incident of bad behavior. You can't take issue with what you suspect goes on between your son and his gf, no natter how strong your gut feeling is . . . and even though you may be right about what you think goes on. She is under no obligation to like you either, but she is obligated to be courteous to you.

If your son is using his cell phone in your presence in a way that is rude, then you can call him on that. What influence his gf may have is beside the point. Your son is totally responsible for his behavior, regardless of who influenced him. If he brings a young woman into your home who behaves badly, you have a right to mention that to him. But it has to be concrete and specific. You can't object to what you imagine he and his girlfriend talk about privately between themselves. (Even though you may guess right about it.) That's their business.

Your son and his gf are a team now. They live together. They are a package deal. To get one, you have to take the other. That's how it goes. Someday, she may be the mother of your grandchildren. You can isolate yourself from all that. Their lives will go on, with you or without you.

Lots of people aren't real fond of their childrens' spouses or S.O.s. But, they work on concealing that, if they want to stay involved in the lives of their sons and daughters. If staying involved isn't important to you and you just have to make it clear how much you dislike your son's gf, you can do that. But your life will be the poorer for it. They will walk away, hand-in-hand, and you won't matter much to them. Then that nest of yours will truly be a barren place.

Your son is not to old to be corrected by you when he's wrong. But there's ways of doing that, without burning bridges. I wouldn't get into futile debates about who said what. People remember things differently. Deal with the here and now.