I think the recent errata on Dominate makes it pretty clear that one can't just flip-flop whether they're an ally or not based on what's convenient at the time.
Mind, I think its a pretty heavy-handed ruling, but applying it consistently it does prevent this sort of debate.
In practice, I don't really want to use the errata as written and would rather leave it up to the DM's discretion as to when a creature is considered an ally of another creature, an enemy of said creature, or when to leave it up to the creature .. and this is a case where I'd rule "You're allies, your friend can be forced to move through your space whether you want him to or not."