Mooney instructor pilot reported a gear collapse after touchdown at a non-towered airport during a training qualification flight in VMC weather. The aircraft skidded to a stop on the runway and the instructor and student evacuated safely.

Narrative

I was doing a flight review for a gentleman in a Mooney. This airplane had manual gear with a Johnson Bar. We were entering the pattern into ZZZ and I watched him lower the landing gear and check by pulling on the bar to make sure it was locked and secure. The student continued the approach at a normal speed and descent rate. The student touched down at the correct approach speed and was not a hard landing; but slightly side loaded. We rolled out for approximately 200 ft. and then i felt the right main landing gear collapse; followed by the nose and left main landing gear. We skidded to a stop and remained on the runway. No injuries or property damage. After further debrief with the student we both agreed that the gear was correctly lowered; and the student reported seeing the single green light that the gear was down. I did not personally check the green light; but i did see the student confirm the gear was down multiple times by pulling on the Johnson Bar to check for security. We do not have probable cause to why the gear failed.