Hmm tough one. On one hand if you are telling her the same stuff that you tell us - well there isn't any wonder she thinks that of him you know? If you don't paint him in a positive light for her, she can't see it herself because she only goes off what you tell her.

Her hatred for him is probably just her protecting you - like you would do for Kailey.

I'd suggest that if you want to improve their relationship - start making out to her like he is the best thing since sliced bread and thats why you can't live without him.
I kinda agree with this dahl. I know its nice to be able to vent to your mum but it sounds like she is trying to protect you when you don't need it. Maybe decide to just vent on here and leave your mum out of that loop.

By the way my mum was a bit the same. She didn't hate my df but my venting about him really coloured her opinion of him. I had to stop talking to her about him.