I think mutual consent is required to designate someone as an ally.
you need no consent from the other since it is subjective and depend of the creature itself, i can think of you as my allies and you can think of me as my enemies, likewise we can be both enemies and allies, one good reason for this is we are enemies and a bigger and more dangerous foes come and if we do not join force we are sure to be dead
Huh. That actually opens up an interesting point for Invisibility abuse. If you define your foes as allies, you can target them with offensive spells all you want and you'll stay hidden. Because you're just spellin your "allies," that is.
still the spell say the spell ends if the subject attacks any creature, and any spell that target or hit one or multiple foe are considered attack, it just say that foe are subjective