Not arguing with you ecmst...maybe I read it wrong, or read too much into what she was saying.
Certainly, if she was diagnosed as a result of screening bloodwork and/or exam at her "preventive" service, the coding would remain the same. However, if during the history and physical, she reported she was previously diangosed with something, regardless of whether she was treated, that diagnosis code must be included on the claim. The insurance company's computer may then kick the claim out because the diagnosis coding is not compatible with the (preventive, not problem focused) procedure coding. The service may still be "covered", but gets applied to her deductible.
If the Doc appeals it for her, they will have to send in her office notes for that day and the insurance company will make the determination of how they want to pay the claim.