Many students who work 35 hours in a shelf stacking role are actually doing so as they want to run cars or travel or go out lots (which is fine but it’s a personal choice)
Or they are eighteen and desperately want to date/socialise outside of their course.
I agree with pps who say that just work out what a reasonable level of “keep” if parents are struggling and go from there.
Often after a certain age retail/service jobs are full of quite immature, “chip on the shoulder” types who (even if they have a degree) have made bad lifestyle choices and will attempt to sabotage/bully anyone moving into more professional/middle class roles.
Maybe ok if he’s doing 10 hours temp and keeping his head down but it’s quite easy to be the recipient of inverse snobbery and get into a bit of a negative mindset which won’t translate well to more professional roles.
Lack of social mobility often isn’t due to the Tories or expensive education. It’s due to social pressure from a low- achieving peer group
(“ooh, who do you think you are with your fancy studying when I had three children by the age of 21 and went to the University of Life! And my husband knew he couldn’t progress in his career, he just went to work down the mines to take care of OUR FAMILY like a good man does! And it’s all these posh peoples fault we’re struggling ”)
I’m also impressed at all these “senior graduate recruiters” who regularly have time in between being power brokers saying “you’re hired/fired” to pop on to a free anonymous mumsnet thread like this to share the benefit of their wisdom 
It reminds me a bit of online dating when you get the “I manage 50 people in a national organisation” type guy (so..... you supervise customer service at Tesco then ...)
I have to go, my PA has a stack of applications where I will make the life changing decisions and anyone who disagrees with me is clearly a waster who will be living off their grandmother at the age of 57