Yeah, but if Kite really can't be saved, does it really matter if they defeat Pitou or not? The battle between Netero and the King will be long over then, and that's really the only one they care about.
Any way you look at it, the job Gon and Killua were given by the Hunters was too keep her from helping the King. And they've done that for the next hour at least(although technically, if they hadn't shown up, she'd have been busy for 3-4 hours).
Now their own personal mission of helping Kite? That's another matter.
At the bare minimum, they should be asking her to explain how her powers work.
I want to say...Pitou will keep her word unless the King gives her a direct order. His command is more important than her life or her honor. If things go badly for Netero, the King may come back and order Pitou to kill them.
That's assuming that you really do get to one-shot Pitou. And some survival instinct doesn't kick in, she instinctively raises her aura to protect herself, Komugi dies, and then you have a full-power Pitou, enraged at having failed her King to deal with.
Better to just let her waste an hour if she'll do it willingly.
EDIT: Basically, what Shinta said.