Sorry, I should have said she's a single mum, not sure that makes a difference.
Well it depends on how much she earns but its plausible to work 25 hours rather than full time if she is meeting the minimum income requirements however once the oldest child leaves school then the UC will be minimal. It will really reduce and could end depending on housing costs.
Her UC will be made up of single person allowance £368.74 per month + £315.00 child element. Then if your friend is renting property then the appropriate housing cost will be added on to that amount, may not be full rent, itl be based on the LHA amount.
If your friend is renting then they will be able to earn £379.00 before UC start taking earnings into account. If your friend doesnt have any housing costs they can earn £631.00 before earnings start being deduced from the UC award at the rate of 55p per £1 earned.
There is an expectation that as a single person with kids of secondary school age that you will be earning 37 hours x minimum wage. If youre earning less than that they will be nudging you to look for more work, maybe not drastically so if you have a job but the expectation would be that youd be looking for more hours.
The exception to this is if you are disabled, caring responsibiliies etc.
To sum it up though yes working 25 hours is probably doable for now for your friend but once the kids grow up I cant imagine theyl be able to afford to live like that.