I can totally understand your situation. It's hard to live an honest life and still work in a corporation and play politics. Yes, it SHOULD be that way, but it isn't. I would work with whomever they paid me to work with...because...they're paying me. However, I see nothing wrong with saying your collaborator has been very helpful while you worked with him/her, but if they wish for you to work with someone else, you certainly can. And your collaborator likely knows if they higher ups are not happy with him/her already. That's not your problem.
Anyway, that's how I'd handle it.