There are places were some spells won't work. Teleport is usually the most common of these.
This is NOT related to killing the Sphynx - the game doesn't care whether or not you keep your oath (drat!). There are no consequences to offing the various nice NPCs in the game, although it does appear that in the original design for the game consequences were planned but didn't get implimented when they rushed the game to market.
Now, back to healing. Have you tried changing his position in the party? What class is he? Is he a vampire? If he is a vampire, healing isn't going to work on him!
He needs BLOOD!!
I've never played a vampire so someone else will have to come along and tell you how to make that work, but I think you need to remove weapons, equip a shield or something in the shield spot and BITE!
Wyv