Heart failure is the number one cause when lasix is used. Heart failure can be because of heartworm disease, a problem within the heart, age or any number of other conditions. Some dogs do well with the lasix and improve for a while.
If it IS a heart conditions (and I'm not sure, based on what you've said) then some vets will also use ace inhibitors.
If you truly want to get the whole story, I would suggest that you go ahead and call your mom's vet. Just let them know who you are and that you're trying to understand. Most vets will be happy to help.
Let me know if you still have specific questions.