Professional courtesy means that cost should never play into taking care of other physicians or their families.
Communicate up front if you intend to charge a physician. If you do not collect the copay from the physician for your services, you cannot charge the insurance company. This is considered fraud.
If you are a physician and you visit another practice and that practice does not ask you for the copay at the date of service but later sends you the bill, you are obligated to pay it per the requirements of your insurance company contract.