Agree with making a UC application because even if the award is small, there's fairly significant extra help available occasionally. Also, can they apply for a promotion, or do extra (bank?) shifts? If they have been on strike, so lost income, their union may have a hardship fund.
The other thing to do would be to do a thorough review of incomings and outgoings. Something must have changed to put them in this situation as they wouldn't have been eligible for the flat if they couldn't afford it at the outset.
This is likely to be the rising cost of living eroding their disposable income, but is it more than that? Have they been using credit to cover the rising cost of things? Is there anything they can cancel or cut the cost of? Eg broadband, mobile phone? A few tweaks could
They also need to correctly prioritise their outgoings. Pay rent, food, travel to work and council tax first. Does their salary cover these things? They shouldn't be cutting back on food to pay unsecured debt repayments for example. If they are in debt, they need help with this and might be eligible for a DRO.
The Moneysaving Expert Money Makeover is a good place to start and will systematically go through all aspects of budgeting, with signposting to increasing income, cutting costs and getting debt help if necessary.
https://www.moneysavingexpert.com/family/money-help/