Cracked Pearly Ioun Stone, Regeneration and DYING

[joep]

So a player wants to purchase the Cracked Pearly Ioun Stone because he thinks this will keep him from ever dying based on this which is from the Pathfinder Rules;
Regeneration (Ex)
A creature with this ability is difficult to kill. Creatures with regeneration heal damage at a fixed rate, as with fast healing, but they cannot die as long as their regeneration is still functioning (although creatures with regeneration still fall unconscious when their hit points are below 0). Certain attack forms, typically fire and acid, cause a creature’s regeneration to stop functioning on the round following the attack. During this round, the creature does not heal any damage and can die normally. The creature's descriptive text describes the types of damage that cause the regeneration to cease functioning.
Attack forms that don’t deal hit point damage are not healed by regeneration. Regeneration also does not restore hit points lost from starvation, thirst, or suffocation. Regenerating creatures can regrow lost portions of their bodies and can reattach severed limbs or body parts if they are brought together within 1 hour of severing. Severed parts that are not reattached wither and die normally.
A creature must have a Constitution score to have the regeneration ability.

My ruling is all it would do is sustain him if he falls below 0 hit points but not has gone below his negative CON.

Is there anything that states that regeneration stops you from actually dying, even if you go to -60 hit points.

[voideternal]

Your citing of the Regeneration (Ex) ability is the rule that allows creatures with Regeneration to stay alive even at -60 hit points.

However, Pearly White Ioun Stone does not confer the Regeneration (Ex) ability.

Ioun Stone wrote:

Regeneration from the pearly white ioun stone works like a ring of regeneration.

Ring of Regeneration wrote:

This white gold ring is generally set with a large green sapphire. When worn, the ring continually allows a living wearer to heal 1 point of damage per round and an equal amount of nonlethal damage. In addition, the wearer is immune to bleed damage while wearing a ring of regeneration. If the wearer loses a limb, an organ, or any other body part while wearing this ring, the ring regenerates it as the spell regenerate. In either case, only damage taken while wearing the ring is regenerated.

Note how the Ring of Regeneration allows you to heal over time, but does not give you an ability called Regeneration. To get Regeneration, you need some effect that specifically grants you the ability. One example of an effect that grants Regeneration is Plant Shape III.

[Codanous]

It also says that it allows a living wearer to heal 1 point of damage per round. This would imply to me that a creature that is below their negative CON in damage is longer a valid target for the ring, nor would say any undead creature wearing it.