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How does the Watcher Shaman's spirit boon interact with prime shot (Warlock and ranger).  

 Watcher Spirit
Spirit Boon: ... In addition, your allies can treat any enemy adjacent to your spirit companion as their nearest enemy.

PRIME SHOT
If none of your allies are nearer to your target than you are, you receive a +1 bonus to ranged attack rolls against that target

Does that mean prime shot actually works automatically with the spirit boon?  Or does the ranger/warlock still need to count squares?

No, I don't think it does.  It works for Quarry/Curse (the nearest enemy you can see) but for Prime Shot I'm not sure it does, since that requires no allies nearer than you - and something can be nearest to you, but still have allies of yours closer to it than you.  You'd still have to count squares.

There's probably an argument for all your allies doing it at the same time meaning that you all treat it as nearest, and therefore nobody counts as closer to it than you, but it would be fairly specious IMO.

Treating something as their nearest enemy says nothing about how far away your allies are.

While the Ranger (and his allies) can treat the enemy adjacent to the spirit companion as his nearest enemy, it won't necessarily mean that none of his allies can be nearer to it than he is. 

Watcher Spirit is a you vs enemy distance relation only, not a global positioning assement.

.....

I now have the strange urge to develop a shaman paragon path that has GPS.

Watcher Shaman.

Watches you.

ALL of you.

Spaced invader is correct.  It helps apply curse/quarry, but not prime shot.

There are small number of other powers that require you to do something related to your nearest enemy: Influence of acamar, dread quarry, and the potion of delusion are all ones where the boon helps.