All gaming groups should have a clear-cut policy for how they treat missing players' PCs. There must be some sort of paradigm that the players are comfortable with. I personally wouldn't like to play in a game where your PC is always at risk of dying, even when you are justifiably absent from the session. At that point, it's no longer really your PC, it's the party's and the DM's.
From the way you described the situation, it seems like your group doesn't really have any policy for treating missing players' PCs. Unless your policy is, and always has been, "your PC is at risk even when you aren't there," it seems like the missing player got a bit of the shaft. If you clearly spelled out that his choices were 1) having his friend play his PC or 2) Have his friend be "off panel" or something similar, then he got what he deserved. If it was unclear what his options were, and there was no prior discussion of what happens when a player misses a session, then I would feel bad for him. Unless there is a precedent for having PCs going "off panel," I certainly wouldn't think to ask if it were an option if I knew I was going to be missing. The logical assumption, if there is no past experience of the policy, is that your character sticks around with the party somehow.
I don't know, having someone missing from the session and then killing him off with little chance of recovery seems quite harsh if the missing player policy wasn't clear.
Our policy is that, if you are gone, your character is "off panel" and gets no XP or treasure. Since our current party consists of 2 spellcasters and 2 front-line fighters, sometimes we'll play a missing player's PC if it is necessary for the group to go on, but that character can't die/be permanantly maimed. With our group, telling someone "hey, by the way, while you were at the wedding, your PC got killed and can't be resurrected," would receive the same reaction as the DM telling us "Ok, you're walking towards town... and an ancient red dragon spots you and eats you all for lunch. Would you like to roll new characters?"
Of course, YMMV, but hopefully no matter what happens, you'll have a clear-cut "missing player" policy for the future
