It's too late monster in law!

My partners mother n I don't really see eye to eye most of the time.. My partner n I have a son whom doesn't really like his grandmother all that much either. He is 5 years old and already tells me he dislikes her. She's not exactly the easiest person to get along with, she's defiantly not maternal despite having 3 children n she's quite self centered more interested in finding a man n not growing old! She's never really been there for her only grandson not has she shown interest until recently when he's started openly saying "I don't like u". She asked my partner if they could all go on an outing but not invite me as she thinks our son will start liking her if I'm not around.. She's done this twice already and my partner goes along with it., is this right??? I always tell my son the importance of being nice to his grandparents n family m he is nice to all of them except for her.. What should I do?