I think Der Kluge's idea is the winner! He'll bargain with them and make a deal. I didn't mention that this is a side-adventure on an adventure path and I'd prefer not to totally reroute the campaign.
It's also a slightly toned-down ogre mage, with only a 5d cone of cold, regen 1 per rd, and magic resistance of 16 instead of 19 (I wasn't being TOTALLY mean to them). Splitting the group has been the real weakness, here.
Anyhow, I think the Ogre Mage will use his cone to make a nice display of power (freeze some of the cave water) and threaten them into listening to him. He'll offer to return their stuff (a wand and a medallion) that he swiped earlier, if they deliver a box to a specific location (in town) and leave it.
I'll figure out what's in the box (some nice distinctive magic item, of course) and who it is being delivered to later (possibly an ally in the theives guild), if necessary!