Final answer:
Upon receiving a subpoena for a client's records, a social worker should first consult with legal counsel or their agency's legal department to ensure they comply with legal and ethical standards while considering the confidentiality of client records.
Step-by-step explanation:
When a social worker receives a subpoena for a client's records from the opposing party's lawyer in a custody battle, the first step they should take is to consult with their professional legal counsel or their agency's legal department to understand their legal obligations and the appropriate response. They must also consider the confidentiality and ethical requirements pertaining to their profession. It's important to evaluate the subpoena for its validity and to understand whether it requires compliance or if there could be grounds to object to the disclosure, always in accordance with legal advice and applicable laws such as HIPAA (Health Insurance Portability and Accountability Act).