Well Its good to hear they are taking him to the vet!!!!! Birds hide their illnesses very well and usally will only show signs of being sick when it starts to get too bad for them to hide any longer...this is the point your friends parrot is at...How old is this bird...I would suggest to keep the bird hydrated to mix in some either gatorade or pedialyte into its water. How old is the bird? And make sure that the bird stays warm , so putting a towel over one side of the cage at all times to where he sits mostly is a good idea this is NOT a good time to get a draft.You could also put a heating pad over the top of the cage with a towel on it, on a low setting for a while, but dont leave it un attended. Also, where does your friend keep the bird in the house? Kitchen? Bed room? Near a window? Their are so many things that can harm birds, most that people dont know about....Teflon cookware is a bad one...The fumes give off toxins to a bird, also maybe the bird got into something thats toxic to him? Maybe chemicals or soemthing as simple as chocolate or advocado? I wish i could help more, get back to me....
Ps. What are they feeding him as a main diet and what does he get for treats.....Could it be a crop infection? Is the bird recently comming off of hand feedings?