small pumpkin man
Explorer
Ally means "willing target that you designate". There's no reason for it to get more complicated than that, game balance or verisimilitude wise.
[Also, in most cases NPCs don't have healing surges to spend, making many of the abilities not applicable even if they could be targetted.]
One other thing ... allowing an unconcious [0HP or less] character to take a short rest will put it back up to 1HP without needing any outside healing.
I would consider an unconscious foe "willing" and therefore an "ally" for healing purposes unless I as the DM judge that the foe's some sort of suicidal fanatic.