Given that the character is out of the action, I would not only give the xp, I would also attempt to include the player in the game in some other way. I might give him a monster or two to run, or ask him to take over condition/status tracking from me (or from the player handling that).
As a matter for the player, he (or she) shouldn't suffer slower advancement from a random bad roll, any more than I would hold back xp if someone was sick and had to leave early.
As a matter for the group, I find that is is a lot easier to work out advancement in the Character Builder and get new sheets printed if everyone advances together, and I know roughly when to tell them to start thinking about choices.
In the "game world", the character who does something dumb and useless in the fight gets xp (I shot Lightning at the Shambling Mound!). The character that sneaks off from the fight (to pre-emptively look at the loot) gets xp. The character who hides and holds back, due to no healing surges gets xp. I think that the guy who got nailed by the drow would have learned about as much as any of them.