The company he works for has repeatedly told him that they can easily live without him, despite the good job he's doing for them (yes, they acknowledged it); though this might be true (well, it surely is) there are some words of caution.
The first and most important is that this attitude can only lower morale and productivity, not only for the person involved but for the whole group, as it can lead to questions like "If that's the way they treat him, why should I be in a different situation?". Someone might think he's different, but the biggest problem when you have a problem is to admit you have a problem (sorry for the pun). And when do you want to know you have a problem? well, now, or at least as soon as possible. That's why good people realize it. To quote from the article,
The good people look for a better place to be, and you end up with what you desire: plug compatible programming units.
Unluckily, unlike plug compatible hardware units, plug compatible programming units are not a great tool.
Moreover, it won't be that easy in the beginning, and it has a cost: studies show how substituting someone can lead to burn up a lot of money, and to get the same productivity level can take up to one year and a half. You even start with a negative productivity, as in the beginning the new resource not only is a sheer cost, but also takes his peers time thus lowering their productivity as well (I will not do multitasking! I will not do multitasking! I will not do multitasking!).
The best way to work it out is an honest and open conversation (I know, I always say so... it's because I firmly believe it). A collaboration must be to everybody's advantage, and everyone involved should make sure that this is so; when it's not possible (anymore) it probably makes no sense carrying it on.