I don't think they'd be insane. They're just brainwashed into believing something; they're not clinically insane.
I'd have a Fort save first, to withstand the physical effects of the torture / brainwashing.
Once the Fort save is failed, the Will saves start. If it takes a while to fail the Fort save, that allows his companions a rescue attempt before the brainwashing starts.
Or perhaps a Fort save, with the DC of the following Will save modified by the relative strength or weakness of the Fort save.