You have gradually provided more information deb1987 which has given a wider picture. Reading through it is possible to see that you have tried to help/advise etc , and you admit that you don't approve of her or her choices; she is entitled to not listen, that is a fact and how she chooses to live her life is her choice, just as staying with her/not staying with her is your son's choice!
So...what do you want to happen now? I don't mean you want her to just not be there or whatever, but where do you want this situation to go within the realities of where things are at now?
She is there. She is your son's partner and your Grandchild's mother. That is the reality! The only things you can do anything about are your own behaviours! How is "not talking to her" helpful to anyone, most of all your grandson? You don't have to have a close relationship but maybe a civil one ...at least on your part! Otherwise things will only get worse and your grandson and the coming baby will grow up ...and maybe not like the way their mum is being ignored.
I really do understand that this is difficult and that working with other people's teenagers is very different to dealing with relationships closer to home! (I know that from experience!!!!) But unless you think about what you can change in the situation (ie how you treat her, approach her etc etc) then nothing will ever stand a chance of changing, is likely to get worse and may well end up with your grandchildren walking away from you. Try to look beyond the here and now to the future and how you can positively influence that for yourself and for your grandchildren.
- hoping the venting has helped.