My rat is ill and I needed to know some dietary needs that could help him.?

I have two rats, one has always been a little friendlier than the other. The friendlier one has suddenly become ill whilst I was away on holiday and a friend was looking after them for me. There was no changes to the diet, but now he has grown skinny and a lot slower. His balance has gone completely and he is constantly falling over. He's also got diarrhea so i gave him a bath and warmed him up but nothings changed :( HELP!