There's no real way to win with the Zaith'isk. Succeed DC 30 skill checks or Lae'zel will suck for the rest of the game. It's better to metagame it = just ignore it or save scum it.
Really odd design choice there to encourage the player to ignore the storyline by punishing them like that. Also, where is the option to just pull her out of the device after you know it's killing her? Why do you only get options to persuade or deceive her to get off? The whole scene feels off.
I just can't get over that the characters have been utterly incinerated in lava and had their bodies crushed to goo by falling into bottomless pits, but somehow only the Zaith'isk hurts so much it gives them a permanent debuff.
"Suck" is a bit strong. Her most important attribute is unchanged, and the rest will very rarely come up. Try using her and see how often she actually fails a save by one or drops because of that exact lack of hitpoints. It might never happen.
Also, D&D games in general are wildly inconsistent when it comes to combat healing and story healing, and they have to be if they want to tell a "traditional" fantasy story while keeping all that magic healing around.
