So I was looking through the manual today...
and just came across something interesting. It's the first time I've ever seen a section of the manual setting down hunting etiquette. rofl On page 37 it states the following:
Often, there will be several ungrouped characters hunting in the same general area. If another player (who is not part of your group) is winning a fight, or at least holding his own, it's generally considered rude to attack his target. Furthermore, such behavior gets you nothing, since the monster has already been tagged by the first player.
On the other hand, if the character is obviously about to take a fall, or is being ganged up on by numerous strong foes, it's usually OK to step in and lend a hand. Of course, if a character runs from the combat or calls for help, that's a pretty strong clue that it's OK for you to take over his combat.
Knowing the difference between an welcome rescue and an unwanted intrusion is a call that will become easier to make as your skill in the game increases.
It's usually fine to heal or buff a stranger in the wilderness between combat. In fact, this is often a good way to make new friends.