If it is indeed tapeworms and not lack of food, she's had to have them for quite a while I believe. I think they take a while to actually make dogs show that much. My three had them before, for about a month (although they were on medication from the vets) and they didn't show signs of weight-loss.

Dogs can get tapeworms from a few different ways..
Ingesting a flea with tapeworm egg in it
Ingesting another animal infected with tapeworm
Ingesting feces with tapeworm

-sigh- Hopefully animal control there keeps an eye on dogs they are called about.