Depends on the social contract of the group. To put it bluntly, no risk, no reward.
If a player expects his character to be present during a session they are not, theyn they can gain XP and can die.
The DM did NOT kill the player. Well, maybe if you have an adversarial DM gunning for them, btu that's a different issue. The DM is a story teller, adventure/plot/world/NPC designer and an adjudicator. They set up a challenge, and then run it fairly. If a character dies, a character dies. Regardless if the player is there or not.
To put it another way, I'm not about to reward a player for not showing by making their character unkillable. I feel that way as player or DM.
Now, player can't make it, tells the DM to write him out for a session - I've got no problem with that. No risk, no reward. It may harm down the road if a player misses a bunch of sessions and is a level or more behind, so I don't want to play in a game with a regular absentee player, but RL happens and comes first.
Good luck,
=Blue