Just because the ally is now participating in archery fire and skirmishes, doesn't mean you can no longer spot the ally. Horn is a maneuver action, so long as you can pay the cost it will have its affect each time you pay that cost. The result is that each time you exert Boromir for the same ally, that ally's strength would go up 3 each time.
I'm thinking I agree with this side. I can't find anything saying you can't do it.
I was sad to see that card Xed. It was good, but I didn't think it was THAT good.
It's funny because people always say it'd be unfair with
Treebeard, Oldest Living Thing or
Treebeard, Earthborn, but the point is moot, because they both have Unhasty.
Tom Bombadil, on the other hand, is another story (though I'm sure it could have been written differently, if Horn hadn't been banned). Decipher said, "There is no cultural enforcement on the special ability of this card. This permanent effectively circumvents the rule of 9 by allowing non-companions to skirmish." I will say that the two Horn decks I made for Fellowship Block are very unpleasant to play against.