
Usually, PDSA will cover anyone who is in receipt of Housing Benefit, and/or Council Tax benefit. So just because she doesn't pay rent, if she is eligible for Council Tax benefit then she should be covered. (I have been in similar situation when living with relatives, so no HB, no rent but qualified for CT ben).
Depending on where she lives, Friends of the Animals if they have a branch in her area, are there to help people caught in the poverty trap. They offer reduced rate vaccination and spay/neut and there is no qualifying rule.
Would just like to say that PDSA although they seem to be a "free" service DO have an up-front charge for vaccs/neutering. And if you go to them for other issues, they give you an "estimate" of the costs of treatment at the end, which appeals for a donation towards it. I have never, even in my most poverty stricken days, ever walked out of there without donating as much towards a half of that estimate that i could, if not all.... (providing of course it didn't leave me without electricity or dog food).
Edited to add:
http://www.friendsoftheanimals.co.uk/may help people outside their catchment area as well.