With congestive heart failure for a dog, it can be the beginning of a lot of issues. For some reason it usually is linked to fluid in the lungs as well. My dog had congestive heart failure and it caused fluid to build in his lungs. There are treatments for it but the dog will usually only last 6 months to a year after diagnosed. And that is if he has treatment.
The treatment usually involves a diuretic that will cause kidney problems and lead to kidney failure. When my dog went into kidney failure I opted to have him put down.
There is a treatment but with his heart problems they said it was a delicate balance and they could not assure its success. Plus they could only treat his kidney and prolong his life for only a few weeks if successful.
Sounds as if they opted not to treat his heart and he probably passed away of heart failure or fluid in the lungs or they just opted to have him put down.