When I was 18 I paid about £350 (out of £1200) a month and was commuting into London on the train. I still had enough money to get wrecked pretty much every night of the week.
Turns out they’d saved every penny and gave it all back to me when I finished uni, god knows how they kept it quiet!
It really depends on her outgoings, transport costs, phone bills etc – You should probably decide a ball park figure for her to have as disposable income to piss up the wall and the rent can be her savings.