I think it really depends on exactly how they handle it. If they simply bad mouth the company and tell the guest that this was all a stupid idea, then yes, I think that would be grounds for discipline. A better way to handle it would be to apologize, explain that it is a test, and that they are seeing that things aren't quite working out as expected. This way they would be admitting that there is a problem, sympathizing with the guest, but not totally trashing the company.
I agree this is the best way to handle it. But there is a not-so-small percentage of guests that will be irate, threatening, bullying, and swearing up a storm that can make it difficult even for the most docile or laid back CM to remain 100%. Everyone has there limits, and in a situation like this, I would imagine almost all the CMs were pushed to them.