Your friend is willing to let us experiment on him? Troubleshoot it like anything else. Can he do shots, can he drink GF beer, can he eat sugar cookies, does it hurt any other time after any other intake of any kind?
Ulcer, allergy(yeast, wheat, barley, gluten, alcohol), pylori, could be anything. Make the Dr. appt, and while he waits for that date do some experimenting.