OK, here goes:
Hand of the Apprentice duplicates Mage Hand (says so right there in the description), with the addition that it can control a weapon well enough to make an attack. The control needed to untie a knot is lacking. You could use it to try to jerk the rope free, but that would still leave you trying to lead Rombard out of the cactus patch by remote control - it's likely you'd kill the poor beast before getting him out.
However, all of this is moot because Harnry is a Conjurer, not a Universalist and so he does not have the Hand of the Apprentice power to call on.
(Come on, you wussies! The cactus won't hurt that bad even if you fail one of your saves . . .)