Kai told Josie she can break her mora miserium and absorb enough dark magic to get her whole family out of the Prison World.
Of course, doing that will either kill her or “turn her into a monster,” which may be why Kai was so willing to help.
Luckily, Josie and Ric held on long enough to escape danger, which was fortunate for Kai as he realized he was duped by his niece with fake Bennett blood.
Whether it was pride in her treachery or just a convenient way for Legacies to give the Saltzmans an out, Kai decided to ring up his niece and give her a tip.