The wording of particular rules can be super-important, but as far as I'm concerned, pretty much every decision is up for group discussion at my table.
I asked my dwarf Cleric what Words of the Unspeaking looked like and he told me that a face formed on stone objects and spoke to him. A few sessions later, he used the spell in desperation while being pursued: he wanted a stone face to grow out of the floor and attempt to trip the pursuer by biting its leg. I immediately said "sure, roll +WIS" because as far as I'm concerned, our shared fiction overrode whatever the spell description said. We established how the spell works: it makes a face grow out of the stonework. It'd be more incongruous to say "but it can't bite" because of some text.
Similarly, if the group decides "ally" means a friend, a brother-in-arms, an ally of your god, or just a target, I think the important thing is consistency- that it always operates that way, and that it defines how his magic works a little more. Plus, asking the player leads to more fun and interesting rationale than "because the rules say so."
"So your god doesn't care if the ally is a non-believer? Why is that? So your god holds you to a different standard than non-believers? How do you feel about that?"