I make it clear to my groups that Exeprience can Vary between characters and that I have several criteria.

#1 Showing up to the sessions. I give experience for attendance. If you only show up half the time, your only going to earn half the experience, even if your character was there being run as an NPC.

#2 Participation. I have had players that don't participate in the game other than to roll dice when ever they are asked. IMO they are barely better than NPCs tagging along with the PCs... I don't have a problem with players who really only come for the social aspect, but aren't fully involved in the game. They just have to realize that their characters won't be as rewarded as the active players.
#3 Indiviual actions. Each character is viewed individually for what they were involved with. If a character ends up going on some sort of solo Activity, like scouting out a bunker, They will get rewarded for that activity becuase they were the only one at risk.

Beyond that, typically if you were involved in aiding the resolution of an adventure, you get Exp for it.

In this case, I don't think the GM did anything wrong, The player choose an alternative that left him out of the "Experience" that the others risked themeselves for. If the player realizes his character isn't a good fit for the group? then let him make a new character with the same Experience his old character has. If he is jsut using that as an excuse and is leaving the game either way? Let him walk. He is the only one complaining and everyone else is having fun so don't let him push you into caving on the issue with the threat of leaving. Else you set a precedent as a GM that you can be walked over to get what one wants.