I've been crunching numbers and can't get to a level of savings that would give tariff income close to the total entitlement of £265.31, and not exceed the upper capital limit of £16k at which entitlement stops entirely.
The first £6k is disregarded, so at £15,999, the assessable capital is £9,999. That gives 44 x £250 or part thereof, multiplied by £4.35 gives tariff income of £174.
Entitlement of £265.31 - £174 tariff income, so there would still be £91.31 of UC payable.
(Disclaimer: I am very tired and have been doing difficult sums most of the day, so I may have gone completely number-blind and be talking bollocks.)
The sooner she starts a claim the better, OP, because she could be "sanctioned", ie have payments stopped for a fixed period, because she gave up her previous job voluntarily.
But in answer to the question about what do people in her position do if they don't have family support, the answer is that they struggle. They have to go onto UC and move into cheap shared housing, because UC won't pay the rent on a self-contained place for single people until they reach the age of 35, unless they're unwell enough to qualify for PIP. Unless they're very lucky, they'll have to use part of the £265.31 they get each month to top up their rent, because the max UC will pay rarely covers market rents.
I think for people in your DD's position, the answer is to take a job, any job, because it's always easier to get a job once you can show you're employable. Look at the public and voluntary sector, where someone bright with a half-decent degree can get an entry level job and progress quite quickly if they show ability.
A colleague's son was in a similar position a couple of years ago, and after a series of pretty shit jobs in retail and hospitality, he did TEFL training. He's teaching English to people from abroad now, and loves it.