I'm the GM in the game which prompted this question, and I'd like to say the way I see it working.
The involved parties are currently in a Stinking Cloud, which provides Concealment(20%) to everything adjacent and Total Concealment(50%) to anything beyond that. So far I've ruled that the concealment applies to the Aid Another check, since it's called out as an attack vs AC 10.
In a related question, how would this be handled in a situation where only one of the involved parties has Concealment? Is the attack vs AC 10 directed at the enemy, at the player being aided, or at a more nebulous area such as a square?
The way it's worded, with "If you're in position to make a melee attack on an opponent that is engaging a friend in melee combat," makes it seem as though it's an attack against the actual opponent, so things such as miss chance that are applied to the opponent would be applied to the Aid Another roll. But that argument could be used to say that the target AC of 10 could change, but since Aid Another specifically sets an AC that's separate from the opponent's normal AC, I'd say that the specific AC overrides the general.
Or at least that's the way I read it.
Yeah RAW doesnt really address that does it. I'd say apply the miss chance if either the target or the attacker is in the fog. The person trying to aid would be the attacker on his roll, even if he's not actually attacking.
In case of other scenarios, he should get only 20% if he's in the first square of fog, 50% if he's 2 or more squares in, in case of reach or ranged attacks