I would probably only allow negative energy effects that specifically say they heal the undead to heal the undead; after all, not all positive energy effects heal the living.
That said, negative energy burst and negative energy ray, both from Tome & Blood, are Sor/Wiz spells that specifically heal the undead. Negative energy ray, a 1st level spell, will heal Mr. Lich of 5d6 damage per casting (given a lich's minimum spellcaster level). Not too shabby; pick up a wand (caster level 9) for emergencies.
Edit: to answer Psion -- I just checked CA & LM and couldn't find either spell in either book.