DA42 Twin Star Flight Instructor reported a gear collapse on the runway during landing.

Narrative

Time of departure - XA:45Route of Flight - ZZZ-ZZZ1-ZZZFlight Included: slow flight; power on and off stalls; steep turns; OEI (One Engine Inoperative) securing up at altitude; emergency descent; Simulated OEI on RNAV XX into ZZZ1; go- around; normal landing with touch and go; short field landing with touch and go; simulated OEI landing to a touch and go; all not eventful and normal. Returned to ZZZ. Upon returning to ZZZ we requested the RNAV XY to a full stop; he configured and then once the plane configured I pulled his left engine to simulate a failure. Student handled the situation appropriately and upon reaching the FAF; the student lowered the gear. Both the student and myself confirmed gear lever showed down and 3 green lights with no red showing. We continued down on the approach and upon going visual for the student; the student lowered flaps to approach at Vref 84 kts; the student and I visually and audibly verified 3 green lights and no red; and made a safe smooth landing slightly past the 1;000 ft markings. Had a ground roll of about 1;500ft. I used my right hand and raised the flaps and applied slight back pressure to help aerodynamically brake; speed checked below 50 kts; then began to apply light brakes as the plane rolled down the runway. Around taxiway 1; I informed the student we would continue down past 1 to slow more and then felt the plane lower and start to slide and turn onto runway XZ-XA at the intersection. Advised tower we were safe then I then began to shut down the plane.Time of Landing - XC:20