Errr edit here due to wrong copypasta. The reason this is an issue is the wording of Aid Another which states:
In melee combat, you can help a friend attack or defend by distracting or interfering with an opponent. If you're in position to make a melee attack on an opponent that is engaging a friend in melee combat, you can attempt to aid your friend as a standard action.
Bodyguard allows you to aid another using your AAO. So, bodyguard triggers AAO when anyone is attacked adjacent to the player allowing you use to aid another, but you can only aid another to defend if you threaten the opponent.
So literal rules seem pretty clear on this, but I don't think it's as bodyguard is intended to be used.