I am writing a story where a person suffers from a medical condition where the gut produces too much of a liquid called diphosphane. Diphosphane is pyrophoric past a certain small concentration, and in the story, a character has something wrong with his gut which causes him to produce too much of this liquid. One day when he is sitting in a chair with two relatives, the diphosphane levels in his gut reach a critical amount - self-igniting and causing all the other flammable materials (methane, hydrogen, and other gasses) in every part of his gut to ignite at once.
What is a realistic outcome of this? What effects would happen and what would the witnesses seated next to him see and hear? Would there just be an uneventful ''pop'' sound, or full on Spontaneous Human Combustion?
Also, assuming that the oxygen in his gut is sufficient. (ie, this condition causes oxygenation of the gut, as well, until it is around 16-21% oxygen.)