depends on your local area
if you live in a city or a large town, id want them to at least consider looking for a job - if they try to find one and cant, need to not work due to educational commitments (eg. dissertation research, internships, work experience, etc.) that is different to them not trying at all.
However, in an area like my own the 'summer jobs' going are 25+mins away as i live in the peaks in an area where there isnt much here... i would end up hardly better off for working, it at all! im lucky that my parents dont make me. Their standpoint is that if im working, i pay rent. If im not working (whilst in HE) then i dont, but im expected to do other things. For example, this easter i have rebuilt two drystone walls and painted the downstairs bathroom, along with cooking dinner for the family every night, taking responsibility for the family pets and making sure the house is clean, and doing any lifts/dashing around that my parents need. It works on a give/take basis here. By all means ask for extra money for food if you need it, but be aware that what she spends at uni may be different to what you spend - if you eat meat every night, etc. and she normally eats beans on toast (bad example, but you get the idea) then the concept of 'youd be paying for food anyway' may not match up to hers, if she spends around £5-£10 max. a week on food at uni and you want a contribution of £20 (for example)!
The way my mother and i get around this issue is that when i go to university she likes to shop in a more expensive supermarket - when i am home we go to Aldi/Lidl together (she likes it but the checkouts scare her, bless her). The food bills come out the same, or actually drop!
YANBU to want some kind of compromise, and wanting her to contribute in some way, even if this is taking pressure off RE: housework, being responsible for certain things in the household, etc. but if you dont know her full financial situation, i would say its BU to assume that she is simply being lazy in not getting a job and has the money to throw around - i rarely go out but i still wouldnt have the money to pay my parents thanks to expenses such as necessary computer programmes, books, fixing broken electronics, food, etc.!