It's Regarding My Friend, He Is Not Able To Eat

It's regarding my friend, he is not able to eat anything, anything he eats vomits back everything, not even liquid food except little water. He hasn't eaten anything in almost 3-4 days. All kinds of blood tests, urine tests have been done, everything is negative and even doctor says there is wrong with him, he doesn't have any disease. His family is very afraid what might happen to him, please, can you tell me what might be the problem and what we should we do ? Please reply at the earliest. It's urgent