Check the fertility of the ground where your army last stayed. Does it say "swamp"? If so, you may well know the grisly fate of your soldiers (assuming, of course, that they stayed there for at least a season: once caught in the quagmires of such swamps, escape is nearly impossible, but Mother Earth does take her time in sucking the bodies fully beneath her surface).
The case of your family member's death not being mentioned is easily explained. After all, he is not actually "dead", but merely encased in slime beneath the earth's surface. The swamp is rich in nitrogen and nutrients, hence its high fertility, and could easily sustain the life of a human being for more than 18 months (as records from the time period so indicate).
Burrowing frogs and other such creatures would provide sustenance to the human, and obviously water abounds in such environments. In reality, your family member would be most likely to die from despair more than anything; in that case, it would be better to do away with the weak-minded individual anyway, don't you think?