All right. Deep breath.
Clearly it is impossible to destroy the immortal soul. It is not a substance. Death is the great enemy - it's difficult to imagine this second death, the death of the soul, insofar as the soul is the essence of a person, and perhaps she should not try to imagine it too hard.
Possibly there is something like that.
More likely - Charon has invented some sort of creative binding and twisting and torturing of the soul.
Imagine, then, that plant souls and this "elf druid" have some sort of faculty, costly to exercise. That could well explain it.
Really it makes little difference, anyway. Charon needs to die still, and healing cannot defile a person still, and those are the concerns of the moment.
...Possibly "soul" just means something entirely different to Griffie. Like "druid". Damn it, she should have caught that sooner, a lot of her attention is elsewhere.
"Do you know how this... ritual actually affected Erloria? What does all this feel like?"