Whats wrong with my friend ?

My friend hasn't been feeling well he was diagnosed with the flu last week his partner took him to get tested to make sure he didn't have Covid he was negative for Covid but positive for flu he's still sick. He's healthy one day next day he's sick can't keep anything down. My girlfriend made us muffins to go with our dinner she gave him hers to see if it'll settle his stomach he threw it right back up. He doesn't even hardly eat much just breakfast lunch and dinner and a snack once in a while. I think we may have to go get him tested again he gets a fever once in a while this is the 3rd time in a week he's been sick. What's wrong with him ?