You won't find coding guidelines for no-charge visits - a no-charge visit isn't coded at all, so there's no right or wrong way to code it. As far as a coder charging for something that a physician has specifically told the patient is going to be a no-charge, I would highly recommend against this because it is not good customer service to bill the patient once they've been told there won't be a charge and it could negatively impact the physician's relationship with their patients. In addition, if the physicians own the practice and are your employers, they would likely consider this to be insubordination.
But that said, if you see that physicians are giving away free visits that they should be charging for, you should discuss these concerns with your manager or the practice owners. There are some potential compliance implications to giving away services for free, especially if it involves Medicare patients. And of course, the revenue loss to the practice involved in treating patients without charge needs to be considered as well. It's best for a practice to have a written policy for when and how no-charge visits will be offered.