A friend has been vomiting a black substance and has terrible pains on her right side. gall bladder or appendicitis?

Neither: Vomiting a black substancde is usually the sign of blood in the stomach or hematemesis. Both gall bladder disease and appendicitis can cause nausea, but not usually hematemesis. This person should be seen by their physician or seen in an er.

Either: Black emesis could be blood or bile, but both could be caused by appendicitis or cholecystitis(gallbladder disease). Right sided abdominal pain could also be either, among other things. Your friend should see a doctor to help diagnose.