I can't talk to the experience of a low income household, so I'm not going to butt in on that.
What I can hopefully do is talk to variety of holiday working opportunities available:
My first Christmas and Easter back home, I was able to go back to a job in retail I'd had part time whilst in year 12 - does she have any previous employers she could contact?
My first summer back home, I didn't actually go back for more than a couple of days as I went to work in Europe with bed and board provided. I used a specialist skill (does she have anything like that? Sports coaching? Performing arts?) but there are also summer camp jobs etc.
Next Christmas and Easter I worked on a project I'd heard about through university contacts doing some (non-academic) research/data analysis - does she have any university contacts or do you have any professional contacts who might need short term projects work done?
Second summer I signed up with a temping agency and did everything they threw my way, from fruit picking to waitressing. I hated it but I hated not having any money more, and I was able to get 5 days work per week by accepting everything.
Final Christmas I was asked back by the same people who I'd worked for the year before for a different project. The final Easter was the only holiday I didn't work (revising for finals).
The summer after university I got a job for a residential TEFL thing in my university town, which again had bed and board provided. We continued to recruit quite late so it might be worth her taking a look at something like this before she comes home/in another university town.
None of this is very long ago, so I'm not hopelessly out of touch with what's available now. It's worth her thinking imaginatively about what's out there, and possibly expanding the horizons a bit. For example, it's often quite easy to pick up a free/cheap old bike (assuming she doesn't drive) which gives so much more geographical scope than if you're depending on lifts/public transport/walking.
As a side note, now I'm involved in recruitment I'm definitely biased towards candidates who have worked through the holidays - no matter what their other background - with bonus points if they've had a "real job" and not just a succession of internships. So it's a long term, as well as a short term, game she's playing here.
A final side note, is it just me that wouldn't have dreamed of not working over the hols, finances almost aside? I'd been in the library for 8 weeks - I wanted to be outside/talking to people/learning about something that wasn't my degree! (plus I suppose I still value extra cash over extra free time...)