I think this is one that, as a GM, its important to discuss with your players before it comes up. Pick a rule: GM plays your character, another player plays your character, or 'fades into background'... and stick with it. Make sure everybody understands what can and cannot happen to their characters when they aren't there. Then you don't have this 'problem'.
My group plays with the 'fade into the background' style; for story purposes, the character is typically described as 'damaged' in some way: disease is okay, but as noted some people are immune; they may receive a concussion, have exhausted themselves with exertion in the previous round, or some other plausible plot-line. We've seen 'Okay, you guys go in the dungeon. I'll wait by the entrance to guard your backs'; we've even seen 'Uh guys, see that rune? It foretells death for any monk of so-and-so who ventures past it. I'll be waiting here.' On board ship, 'seasick' strikes me as both plausible and realistic: there are some people who never DO get their sea legs. Occasionally, the GM uses the absent PC as an 'NPC voice', letting him/her give the party a hint or two. For long-term absences, the PC disappears - even IF we're in the middle of hostile territory.
a player who doesn't want his PC to have any chance of dying could just not show up to the next game be assured of invulnerability.
Sure, but if absent players don't earn XP or treasure, then he is sacrificing all chance of 'gain' along with all chance of 'loss'. The player will quickly see how they are falling behind in XP, and realize that they need to appear. (Honestly, I've never seen this happen, and wouldn't want to play with a player who thought that way.)
For your particular situation, you have a number of options - new character, revive old character, etc. If the player is really chapped about it, I'd say bring him back to life, with a debt (to whichever entity raised him); if the group were between quests, its a great opportunity to lay a
geas on him. Emphasize that its the last time you're going to 'rescue' a character in that way, and outline what the rule will be for the future.