They might have used Rhue's illness in story. I'm trying to decide if a Mcgivers crippled from trying to remove the eel along with a dead son would have been even more powerful and dramatic motivation for Khan. Her death in the final battle would certainly have moved Khan to set off the Genesis Device instead of finding a way to fight on.
An interesting idea, although it comes perilously close to exploitation. Perhaps Bennett and Meyer didn't feel comfortable casting Rhue under the circumstances. It's also possible that she was too ill to participate, so her inclusion would not have been possible regardless of the circumstances.
--Sran