Stogie, it's good to have you here. If your dad starts retaining fluid again, he may need to go back on at least one diuretic. I am on both Lasix and Aldactone. The problem is that Lasix depletes the body of potassium, which can be dangerous to the heart. So the Aldactone helps prevent that, and is also a mild diuretic.
If your mom can manage to eliminate beef, it would help a lot. He can still eat poultry and fish. I get ground turkey and mix it with A1 Sauce. I then grill it on my contact grill and I swear it tastes like a regular burger, especially if I use ketsup on the bun, though I usually use cranberry mustard. She can also up the amount of fruits and veggies, fresh if possible. I don't have diabetes, so I don't know how much fruit he could have without it playing havoc with his sugar levels, but I guess it would take some experimentation. If he absolutely insists on having beef, just try to make it no more than once a week. It's the hardest thing for the liver to process. However, you are correct that we need protein, and there's no reason to go completely vegetarian. I do more with beans in the winter and also eat a lot of ratatouille (a vegetable stew), but in this warmer weather I don't want to be bothered. It would be a good time for her to make more main meal salads, though, with grilled or roasted turkey or chicken strips. Just a few suggestions.
The transplant process is such an odd thing. One must be sick enough to need a transplant (REALLY sick), but at the same time healthy enough to go through the surgery with a good chance of survival.
Just keep educating yourself. Knowledge is power!