I don't have a paladin in my party, so it hasn't come up... but I'd be inclined to make the DC scale with the number of points he's curing.
Something like 15 +1/5 points, or +1/8 points.
If you're healing 10 points of damage, that's more like a Cure Moderate Wounds than a Cure Light Wounds, so the DC should be slightly higher IMO.
But since there are no fixed rules on the subject, it's up to the DM.
Isn't there a ruling somewhere that he doesn't provoke an AoO if he's using it to damage undead instead of heal?
-Hyp.