First, the other contributor who answered you is not, to my knowledge, licensed to practice law in California. So, if you are requiring an answer from a California attorney, then you may want to "opt out" the other contributor, and wait for someone else from California to answer.
That said, California has very special requirements concerning the release of records which are inherently confidential, such as medical records. Also, the rules are slightly different where the subpoena requires a nonparty witness (e.g., physician) to produce records at a hearing or trial, as opposed to production prior to trial (i.e., during the discovery phase of the case). In either circumstance, the person sending the subpoena must provide special notice to the party whose records are being subpoenaed, and give that party an opportunity to object to the production. Code Civ. Proc. §§ 2020.510(c), 1985.3(e).
The other party must then file a motion to quash the subpoena, in order to avoid production of the subpoenaed documents.
If this does not occur, then the subpoenaing party has the option of either filing a motion to compel production, a motion for contempt, or a separate civil action to recover $500 plus direct damages for the failure to comply.
So, the basic answer is that the records must be produced -- however, physicians generally are able to assert doctor-patient privilege, and that could permit the physician to avoid sanctions for failure to comply.
Concerning opposing counsel's letter, that could be viewed as an obstruction of a court order -- or it could be an attempt to avoid an unlawful production of the subpoenaed records, on grounds that the subpoena is not in compliance with the special notification rules.
As you can see, the answer is not nearly as simple as it may appear.
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