Guinea pigs do not require that much space. It would have nothing to do with why the pig is sick in the first place. A lot of people keep them in mediumed sized rubber maid storage containers, and pet shop cages, and they are fine. A lot of cavy breeders use whats called a condo( stackable cage unti), that have only a few feet for cage space. They are fine.
There is some decent info on the links provided above once you get done sifting through the antibreeder propaganda(think cavy spirit has most of it). However, beware that if you join them and ask a vet related question, you may get jumped for not taking it to the vet first.
I would make sure she is getting water soluable vitamins in his feed and water. The pellets should be enriched with vitamin c. You can buy Vitamin c drops to add to the water too. I would also try to seek out the advice of a knowledgeable cavy breeder, or vet. If she's old, then his immune system may also be weakening.