The player is making a Death Attack with a poisoned (injury) blade and wants the poison save to happen before the save from the Death Attack ability. As far as I can tell, these effects are simultaneously triggered by damage. His poison does Constitution damage. So he is hoping that a failed save on the poison will make the Death Attack save harder.
Death Attack (Ex): If an assassin studies his victim for 3 rounds and then makes a sneak attack with a melee weapon that successfully deals damage, the sneak attack has the additional effect of possibly either paralyzing or killing the target (assassin's choice). Studying the victim is a standard action. The death attack fails if the target detects the assassin or recognizes the assassin as an enemy (although the attack might still be a sneak attack if the target is denied his Dexterity bonus to his Armor Class or is flanked). If the victim of such a death attack fails a Fortitude save (DC 10 + the assassin's class level + the assassin's Int modifier) against the kill effect, she dies. If the saving throw fails against the paralysis effect, the victim is rendered helpless and unable to act for 1d6 rounds plus 1 round per level of the assassin. If the victim's saving throw succeeds, the attack is just a normal sneak attack. Once the assassin has completed the 3 rounds of study, he must make the death attack within the next 3 rounds.
Injury poisons are primarily contracted through the attacks of certain creatures and through weapons coated in the toxin. Injury poisons do not usually have an onset time and have a frequency of 1 round.
What is the proper order of resolution of these two effects?