The general overarching principle with personal health information laws is that the info belongs to YOU as the patient. It is not for the provider to make decisions about whether or not the insurer could be trusted. Your wife's consent is the key element here, and it does not sound as though it was at issue at all. If the MD had misgivings, he could have asked for a release or some such instead of simply denying your wife's request and putting the two of you out.

I would point the finger at the physician, not the insurer. I would expect that a requirement of proof from a physician is a basic term of Allianz's insurance contract - you will have a hard time ever getting around that, because otherwise what's to stop people from just making claims up? Depending on how much $ is at stake, you may have something worth pursuing as a small claim against the MD or the clinic.