He actually wasn’t comatose; he was conscious, just severely brain-damaged, and masturbated constantly. The injury (due to a piece of shrapnel that was inoperable) gradually became worse, and I believe Jenny performed the act with him after he lost control of his hands. Clearly not capable of consent, but it still seems like something of a kindness, compared to screwing someone who’s unconscious.
Sorry. I just love John Irving.