Most importantly, in your situation, I think it comes down to: is he disrupting the game; do you (or the other players) want him gone; does he want to leave; or can everything work out the way it goes?
As for my game, I follow the following self-made rules:
First, I don't make my players choose the game over other aspects of their lives. Sometimes people go out of town, play in a softball tournament, or out to other group functions as they come up. So, we inevitably play a player down once a month (we play weekly). However, my players are dedicated to the game, and are all engrossed in their characters, so they do all they can do deconflict any other activity. We often switch game days depending on everyone's schedule.
Second, I don't like any of the players being behind in levels, so I try to keep all the players close together in xp. I award full xp to each character, present or missing, but no bonus xp for role-playing opportunities that may have occured. This has kept everyone within 1,000-2,000 xp of each other.
Third, I have to say that I have pretty good communication with my players, and I have known whenever any player would miss a game. Now, what I do to the character depends on how long the player will be gone... one day, two weeks, or more.
If a player is gone for one session, someone who knows the PC well runs him, and we often make group consensus on what the PC would do in any given situation. (It's actually funny that the group always picks the path the player would have in the situation, when the game was described to the player. You just get to know a person and his character after a while.)
If a player is gone for more than two weeks, I send his character on a "side quest" that I discuss with the player. It usually involves character background that the player made at creation. We decide what mission he does, and I let the player describe it to the group when he returns. Of course, everyone understands that none it was roleplayed, but eveyone truly enjoys the stories generated for the players. It adds a bit of scope to the game when the players are able to think of any of the party members doing something other than their current mission... there's more to the world than what's at the game table.
But, if a player were to miss every other (or third) game to play in another game, I would see if we could work out gaming days to help (I do have two of my players in games run by other people I know), and if they simply enjoy the other game better, I would suggest that they simply pick one and stick with it.