So sorry about your friend's sudden and very sad loss of her monitor. I have zero experience with monitors though, & a necropsy would be the best way to
find out what happened. The large black spot on his belly sounds as if he may have "landed wrong" as you said...it sounds like internal bleeding. He may well
have had parasites but it's unlikely that he'd have died suddenly like that from them. What an awful experience, it sounds like she was taking good care of him
too, though considering the information about parasites, a sooner vet check up would be in order if she tries again with another one.