I understood it that if she rents a run-down place it's because of the cheaper rent and by doing the work the rent stays that way and she is less likely to be turfed out. Looong ago when I was a student and shared a flat with other students we kept the rent low exactly by doing that, painting the place, fixing bits and pieces.
Also we don't know her age. She is not a tradie, just handy. To do a full day's labouring on a construction site can be very heavy-going.