I have one such friend. He has had the same girlfriend in the 4 years I've known him, and she is always put before everything else. I want to say "even himself", but that is impossible for me to know - after all, I should think he enjoys her company more than anyone else's.
The thing is, until half a year ago, he'd spent almost all time with her. Sure, you could arrange something with him, no problem, but 3/4 times, he'd bail out on us, on account of him having to do something with her.
Two things though - she's a very sweet girl, and not at all demanding. I don't think it's that much her demanding that he always be there at her whim, but him feeling a great sense of importance that it is so. My theory is, that when she says "I wish you were here" she doesn't expect him to come running, and would perfectly understand a "Well, I'm just in the middle of a game of warhammer, sorry", but he automaticly puts her first.
Where it get's extra tiresome, is when you try to plan something long time ahead. It seems he automaticly assumes that all time should be spent with her - and only knows like a month ahead when he'll be free. Both of us really love Coheed & Cambria. I introduced him to this band 3 years ago, and he was crazy for it. Still is. Now, long time ago, I guess a year or so, it was revealed that they would play two shows in Denmark. There were six months to the show. He said he couldn't make it, that he had to do something with her that day. I really were dissapointed, not in him, but just that it wouldn't be as cool an experience without him, as he was the only friend I had that were into it as much at me. Still, if he had plans, I couldn't blame him. Myself, I'd changed my plans so I could go to the concert, as there was no way of knowing when and if they would be in Denmark again, but still, can't really blame him for that. I bought a ticket anyway, of course.
Some months went by, and it was sold out. I teased him lightly about how great it was gonna be. Then, a week before the show, I happened to win some free tickets. I ask him again, if he's sure he can't come. And suddenly, he can.
I can't know this for sure, but I think he never had anything planned in the first place. He as just afraid to make long term plans for a weekend, that he might be required to spend with her.