Happens to everyone. Your wizard fails his save against feeblemind, and thinks bull-rushing the ogre is a good idea...
Oh, you mean you killed someone else's PC.
Welcome to the club. My personal experience involved a bizarre chain of occurrences which left me thousands of RL miles away from the TPK I caused. Suffice it to say, I'm no longer allowed to design encounters and villains in absentia anymore.
Anyway, you can either
A) Introduce a new PC. This may be temporary or grow to become the player's new character. There a million and one ways of doing this, ranging from "convenient coincidence" to "powerful prisoner", and the limit is basically your creativity. If there's any way to get them to temporarily have the same goal as the others (hunting the same kobolds, or something like that
), you have an out whether or not the original PC gets resurrected. May or may not be a level lower than party average - depends on your group.
B) NPC upgrade. Suddenly Biggs is flying better, and grows plot armor.
C) Give him control over some monsters.
E) Have a coincidental scroll of raise dead / magical artifact/ high-level and extremely frail cleric just around the corner. Perhaps introduce some complications (must serve Pelor in a quest, sort of thing. Sky's the limit)
F) Give them control over animal companions, familiars, cohorts, and other miscellaneous pseudocharacters
One issue is having stats for A/B during the same session the PC dies. Google (or unused NPCs) to the rescue. As long as the character sheets represent vaguely the same type of guy, there shouldn't be much in-game dissonance between Hawk-eye the imprisoned half-elf fighter 7/ ranger 3 and Hawk-eye the human fighter 5/barbarian 5.
Arcane spellcasters are more difficult, due to spell selections.