You can pay her repayments if you want, but in terms of changing the ownership the lender would have to agree it - which is why you need to speak to them. Also if you want to buy in the future, this could reduce your available potential mortgage since you effecively are already paying one...
Also what would happen if you and your girlfriend split up. It could get messy if you effectively had a charge of 20% of her mother's house and the split was mutually acrimonious.
You need to talk to the lender and a lawyer (and her mother).