General aviation FBO reported a safety violation when a Cessna 150 renter pilot reported a mixture control maintenance discrepancy during pre-flight; then flew the aircraft in an unairworthy condition. The aircraft was grounded after the flight and repaired.

Narrative

Subject: Incident Report - Operation of Aircraft X (Cessna 150) with Known Mixture Control Discrepancy on day 0We are submitting this report as the owner and operator of our FBO; based in ZZZ . We provide aircraft rental services for time-building pilots under Part 91 operations. This report documents an incident involving the improper operation of Cessna 150 aircraft X on day 0.On the morning of the flight; at approximately XA:24 hours; the renter pilot; Person B; informed us via text message that the mixture control did not appear to be functioning normally during preflight. Despite this notification -- and without waiting for our maintenance staff to inspect or clear the aircraft - Person B proceeded to start and fly the airplane; conducting a flight of approximately 0.9 hours.After the flight; our A&P mechanic inspected the aircraft and discovered that the mixture control cable had become disconnected from its retention mechanism; a condition that would have rendered the aircraft unairworthy before the flight began. The aircraft was immediately grounded and repairs were carried out.This flight was operated despite a known mechanical discrepancy and without confirmation of airworthiness; which may constitute a violation of 14 CFR §91.7 -- 'Civil aircraft airworthiness.'Actions Taken:Aircraft X was immediately grounded and repaired following the discovery.Maintenance logs were updated to reflect the discrepancy and repair.The renter; Person B; has been banned from further aircraft use pending investigation.All other renters and staff were reminded that no aircraft may be flown with a known or suspected mechanical issue until inspected and released by maintenance.Internal documentation and text message records of the incident have been retained.We are submitting this report voluntarily and in good faith to ensure full transparency and to protect the safety and integrity of our rental operation. We respectfully request that the FAA review the matter and provide any additional guidance or follow-up actions that may be required.Please contact us directly if any further information or documentation is needed.