Hello, I am so sorry to hear that your piggie is sick. Pneumonia can be tough for a piggie and I will pray that she makes it. The best thing you can do for her is to keep her in a very quiet place and follow exactly what the vet has said. The fact that she is still eating is a good sign. This is very important.
Dehydration can be devastating.
If your pig will not drink water, you can get a dropper or syringe from your vet and they can tell you the correct way to administer fluids. Sometimes a pig will lick water or cranberry juice off of your finger if they are too weak to drink. The trick is finding something that they like. Romain lettuce has a high water content and is ok as long as you do not give too much. It can cause diarrhea which can worsen the problem.