potter6
I think it's very important - and healthy - to maintain a good relationship with yours son's former partner when there are children involved. If they become aware that there are bad feelings between their parents and between other "sides" of the family, the split loyalties this engenders causes great guilt and distress to children.
Aside from that, why on earth should you cease being friends with someone who is still an important part of the family? She is the mother of your grandchildren and you are fond of her.
If you can say with hand on heart that you have been friendly and welcoming to your son's new partner and have not given her cause to feel like "second best" then I think it is unfair to say you are being disloyal and disrespectful. It is his new partner who needs to grow up a bit and accept that you all had a life before she came along. She entered into a relationship with a man who has children, which introduces a new dynamic to all his other family relationships.
She sounds rather immature and very rude to me. Judging from your OP, I think you sound like a person who can maintain good manners and a hand of friendship even when others, including your son, are being unreasonable. I do have great sympathy for you in this distressing situation and hope that, in time, things will sort themselves out.