My girlfriend's mother is resentful of me :/?

Hey, me and my girlfriends mother have a good relationship e.g she is sending me to London on a small break with her daughter as a good gesture. She is very nice to me, but she doesn't like my girlfriend planning things with my family or learning anything from my family. My girlfriend said it's not my fault and her mother loves me but she is resentful towards me because my girlfriend is going away to Spain for two weeks and she will just speak of me, she is spending her free time with me and not with her mother. My girlfriend is desperately trying to go out with her mother but she just ignores her or flips out on her because apparently I am top priority. My mother joked to hers saying 'I will be teaching her how to cook soon ;D', her mother got home and went nuts on her because she was jealous, even though my mother was joking, My girlfriend's mother is putting a lot of stress on our relationship and it's annoying me, I can't confront her because she will go nuts on my girlfriend. I chose to take a year off before going to university this year (to tie up loose ends and progress my relationship, before definitely going next year as it's planned), my girlfriend wants to go to Cardiff uni a year after me then, but her mother won't leave her go even though she was the one who came up with the idea of her going. My girlfriend is really stressed out with GCSE's already and with this it's really hurting both of us :/ What can I do?