What do you feed a dog who is a picky eater?

Try clear liquids like beef broth. At the outset, the real problem will be keeping the dog hydrated. Nothing lives long without water. Broth will do (and don't use the Sodium Free broth -- the dog will need electrolytes).

Try varying the dog's diet. If they like steak -- until this passes, get them a steak. Try and find something they'll eat, and then feed them in moderation until the problem passes.

If the dog has diarrhea, this problem is more acute, as the animal is likely expelling more water than he's taking in.

If you absolutely can't get them to eat, and especially drink, it's time to see a Vet.