The most common side effect is dehydration. As long as Poppy eats and drinks sufficiently, she should be ok.
The drug can also cause electrolyte imbalances and kidney problems. Usually though, the benefits outweigh the risks.
Since Poppy is receiving hospice care and furosemide is used to keep her more comfortable, you shouldn't worry about the side effects too much. It's very likely that her abdomen looks less swollen, since the drug reduces the fluid.
Hugs and kisses from Heidi and the kitties.