If a digesting engulfer eats an e monster that explodes (either actively or upon death) or potentially any monster with an explosion attack, the monster is assumed to explode inside the engulfer, who takes double damage from that attack (or 8 times damage, if the explosion is considered to have hit them 8 times) and is stunned and immobilized for a few turns, unless they resisted the explosion’s damage type in which case they only suffer that for 1 turn.

This should also extend to other explosions inside engulfers (most of these would be player-created: e.g. reading a scroll of fire or breaking a wand while engulfed).