Well she's going to have to suck it up, now, won't she? You are the one in a relationship with your SO, not her, not her mother. You're going to have to learn to stand your ground. If someone you consider a friend starts putting your child down, will you let them? I'd hope not. Then why would you let them put your child's father down, if you don't agree with what they say, stand up, tell them, and if they're not happy...
Well good riddance, I say.