well, there's no indication Kiri actually cures anything. Even in chapter 5, look carefully at how he explained it. He didn't cure a cold, he merely boosted a person's immune system and they got over it faster. No one has ever recovered from Troi, and as far as we know there's no natural way to fight it.
What I meant by mentioning Elle's age 20 thing is the fact that her body didn't hold out as good as it was expected to, so she's already on the edge. If Kiri's ability is just boosting, then he may have only bought her time, not "stopped the seizures forever."
Well, although I'm sure my logic is sound, for the sake of the series the author wouldn't dare off a major character like that. But, I see the strong possibility he'll be able to do something like make her go into seizures while holding Kiri's hand, or maybe start disappearing without seizures--something way later on--to add a good element of suspense to the comic. The only solution I can foresee then would be dependent on them finding another special power user. (does this make more sense than what I wrote above?) Just my 2 cents.