she's paying rent; she's just paying it to her boyfriend so he can put it in his pocket. there's no way a young guy is going to let his girlfriend stay there for 5 months while he pays for her accommodation.
you can't ask for rent for the last 5 months (or you could but you're not going to get it), but you can going forward. ask your flatmate how long she's planning to stay and just say it's not fair you have to share your space, water, utilities, with someone who's not contributing to paying the bill.