As others have said, sometimes working part-time just isn’t an option depending on the course. I went to uni about 14 years ago now to do a STEM subject and I had about 40-50 hour weeks, more during exam season which was twice a year. The university told us that trying to work on top of that was the most common predictor of failing. That’s because it’s not like a manual job where you can physically push through to work longer hours, it is all mental work and the hours can drive you mad. I worked harder back then than I’ve ever done in any real job since.
I did a bit of work over the summer breaks to supplement what I had but during the year I had to just survive on maintenance grant. I got the full whack grant because my parent’s income was very low. Honestly, don’t know how I did it looking back but I remember a lot of Tesco value packs of noodles were involved and maxing my student overdraft and credit card. I suspect it’s even worse these days with current cost of living. Whether you live at home or not makes a big difference, I was in Edinburgh so nowhere near home. It was character-building but honestly a pretty awful lifestyle.
In answer to your question, I was living on about £10k a year I think. Rent and bills were most of that. I think I remember working out I had about £50 per week and I just about survived (this was 2009). Based on that I would ensure they have at least £100 pw after bills with the way things are now if they are living away from home otherwise they might come home with scurvy.