It's up to you though, whether you are organised enough to work around it and whether you can deal with the stress of year 12.
It would probably help if you went for scholarships.
It sounds like your friend works a casual job. Casual jobs have a pool of people that are organised via a changing roster or need for workers. Part time jobs usually have a fixed time each week you work (like a full time job but with less hours). Casual workers usually get more pay, but no paid leave or other benefits. Their ability to get a shift is also less reliable than a part time worker's.