Using geas to have a creature commit suicide is invalid
The 4th-level Sor/Wiz spell lesser geas [ench] (PH 235) and the 6th-level Sor/Wiz spell geas/quest [ench] (PH 234-5) mandate that the caster either order the target "to carry out some service" or order the target "to refrain from some action or course of activity." An order to commit suicide certainly "compel[s] a creature to kill itself," which the spell can't do, and an order to refrain from an activity like Never not leap in a pit will certainly cause the target to "perform acts that would result in certain death," which the spell also can't do. The spell will fail if services like those are stipulated or actions like that are constrained.
The 1st-level spell hypnotism [ench] (PH 242-3) and the 8th-level spell sympathy [ench] (PH 292) don't change the spell geas/quest, and, likewise, those spells can't force a suicide.
In fact, many enchantment spells have provisos against self-slaughter:
- A creature affected by the spell charm person et al. "never obeys suicidal or obviously harmful orders, but it might be convinced that something very dangerous is worth doing."
- A "request must be brief and reasonable" if made of a creature affected by the spell hypnotism et al.
- "Asking the creature to do some obviously harmful act automatically negates the effect of the spell" suggestion et al.
- "Obviously self-destructive orders are not carried out" by a creature affected by the spell dominate person et al.
Layering on a creature these spells that specifically say they can't be used to force a creature to commit suicide doesn't make the creature any more likely to commit suicide.1
"But this player's making this argument..."
In a sense, the player who argues that suicide is subjective is correct—for instance, a creature wearing a ring of feather falling that's ordered to jump into a 200-ft.-deep pit isn't being ordered to commit suicide, but, if the same creature is first ordered to remove the ring, the creature is being ordered to commit suicide. Therefore suicide can't be weighed purely objectively.
But it sounds like the player's arguing that this subjectivity makes any order of suicide instantly a valid order because no order is suicidal until the order leads to the creature's actual death. Hence, a creature should follow all orders because the creature's factually incapable of proving the order is suicidal, and the creature will remain incapable of proving a suicidal order until the creature dies.
I am certain students of rhetoric or formal logic have a name for that particular technique or fallacy; however, I am neither, so I think I'd just tell the player to shut up and move on.
1 This is, also, a huge waste of resources. Why hasn't the fighter just bull rushed this dude into the pit already?