It's different because it's someone's mother, not a contemporary.
It's different because she's stayed two weeks so far, not just a night or two.
It's different because legally and officially she is not supposed to be there, so no-one would look for her if there was a fire.
It's different because the mother turns off the oven when the girls are trying to cook something.
It's different because the mother comes out and shouts at the legal, rent-paying flatmates when they are talking in the kitchen.
It's different because the other girls have spoken to this girl to ask when her mother is leaving and she replied that she didn't know.
I would be reporting this both verbally and in writing to all and sundry - Reception, Accommodation Office, Student Welfare, Campus Security, Head of whatever course she's studying...
I would keep reporting it until someone took some action and evicted the mother. And i would demand a rent rebate for every day they didn't deal with it after I reported it.