I am picturing a non-instanteous transmutation, as seen in Harry Potter etc… (DnD would also count spells like “true polymorph” having a casting time of 1 action (~6second).) Not a barotrauma specialist, so can’t comment on instantaneous case.

A particularly healthy set of lungs contains 4litres (5 grams) of air, assuming a conservation of mass, that’s less than one ml of silver, being a noble metal, the silver harmlessly disapates into the theifs bloodstream over the next couple years, at worst turning them blue. The awakened mushroom sorcerer hidden in the theifs pocket casts an actually good spell, such as fireball.