The question that must be asked is:

1) Are you contracted with the Medicare HMO?
If not, you should have obtained authorization to treat the HMO member. Because you did not, and this was not defined as an emergency, you lose. You tell the doctor if he is not contracted, he must get permission from the HMO to treat the HMO patient. The HMO has every right to deny authorization and bring in a network doctor. Doctors need to know the rules of the game and to play nice with others. Being a doctor no longer means you can treat anyone you wish. You are at the mercy of the rules of the HMO. Now, as I stated, if you are not contracted, the rules of the HMO do not pertain to you and as such, the HMO is not obligated to pay a non-contracted provider.

2) If you are contracted, then you need to see what the terms of your contract state when treating an HMO patient. Your contract may require you to obtain authorization. If it does and you didn't, you lose.