How do I ask my mum to let my gf stay for a week?

I know its a large think to ask (provisions, meeting her, dealing with all the other stuff) but her mother has gone bats**t insane and she cant stop crying (shes a strong person too). Problem is homes a minefield for her and she doesnt have another place to go. How should i ask for my mum to do a big thing and take her in for a small while?