It depends on all manner of factors and on here you'll get answers that range from 'I could never charge my child to live in their home' to 'I've lost £x in TC now they're a working adult and I need the money to pay the bills' and everything in between.
She should definitely be paying her way - think about how much she costs in terms of food, laundry, hot water for showers/baths, the second adult for council tax purposes if you're a single parent, that sort of thing.
If she's working full time on a proper wage, she's probably got quite a significant sum coming in, so she certainly should be using a decent chunk of it to contribute towards her living costs and not spending it all on luxuries.
Maybe think of it in roughly quarters - a quarter for board, a quarter for her essentials - transport to work, mobile phone, a quarter for luxuries, including if she spends a lot on clothes, has a car on PCP, a high end mobile - if she has expensive/nice versions of these, they're beyond 'essential' and a quarter to save for a house deposit/moving out/other bigger purchases. That way she'll still have a decent chunk of spending money but she won't have loads to be spending on anything she wants without thinking about it.
But it depends on what she earns. There's quite a difference between 35 hours a week on NMW for an under 25 and a graduate salary of £25/30/40k+. Also her essential expenses, eg if she works shifts in a job 20 miles away so can't use public transport so needs a car compared to her job is a mile away and she walks there.