Maybe just have a quiet word with the foster co-ordinator of the shelter. I think they would be most interested to know their carer needs to be brought up to speed on preventatives. Perhaps even provided with some.
re: heartworm. There is early stage, which can be treated long term with one of the ivermectin drops (can't for the life of me remember which one now).
Then there is later stage, the treatment is harsher and must be administered asap by the vet in a series. Apparently only the daily heartworm tablets are dangerous to dogs already positive to the parasite but I would suggest a test first just to be safe.
Worms: a good allwormer now, and again in two weeks. Then once every three months if it's not covered in your flea preventative.
Sorry to bust in. I don't post here a lot.