again, i must say that for an assured shorthold tennancy, it is for a period of six months (or 12), and then usually plops over into a rolling contract whereby either side just gives the required notice to quit.
However in the first six months it is usually fixed unless the LL can get someone else to move in in which case thye may give some leeway.
even if your friend had not paid the money up front, she would still be required to pay it in the first six months even if she wasn't living there. She really just needs to read her contract