What happened
The FAA’s narrative, in sentence case. The FAA writes it in capitals and removes private details.
Student & instructor were practicing landings. This landing was with the left engine power near idle to simulate an engine out. Upon landing and roll-out, the right main gear slowly began to collapse to the point that the aircraft started to slide on the right wing off into the grass. Both pilots reported that the gear indicated down (3 green) and the air traffic controller on duty reported that the gear appeared to be down, and that the landing was normal. After recovery and inspection the right main landing gear would not fully lock in place. Further investigation found support tubing on the landing gear that was slightly bent, not allowing the down limit or "j" lock to fully engage. A quarter turn was required on the extending push rod adjustment to engage the down limit switch and a full turn was required for the "j" lock to fully engage. It could not be determined if this condition was pre-existing, but appears to be the most likely cause with the available information. Note - the aircraft was substantially damaged in the recovery process, as it was dropped twice.
As the FAA filed it
STUDENT & INSTRUCTOR WERE PRACTICING LANDINGS. THIS LANDING WAS WITH THE LEFT ENGINE POWER NEAR IDLE TO SIMULATE AN ENGINE OUT. UPON LANDING AND ROLL-OUT, THE RIGHT MAIN GEAR SLOWLY BEGAN TO COLLAPSE TO THE POINT THAT THE AIRCRAFT STARTED TO SLIDE ON THE RIGHT WING OFF INTO THE GRASS. BOTH PILOTS REPORTED THAT THE GEAR INDICATED DOWN (3 GREEN) AND THE AIR TRAFFIC CONTROLLER ON DUTY REPORTED THAT THE GEAR APPEARED TO BE DOWN, AND THAT THE LANDING WAS NORMAL. AFTER RECOVERY AND INSPECTION THE RIGHT MAIN LANDING GEAR WOULD NOT FULLY LOCK IN PLACE. FURTHER INVESTIGATION FOUND SUPPORT TUBING ON THE LANDING GEAR THAT WAS SLIGHTLY BENT, NOT ALLOWING THE DOWN LIMIT OR "J" LOCK TO FULLY ENGAGE. A QUARTER TURN WAS REQUIRED ON THE EXTENDING PUSH ROD ADJUSTMENT TO ENGAGE THE DOWN LIMIT SWITCH AND A FULL TURN WAS REQUIRED FOR THE "J" LOCK TO FULLY ENGAGE. IT COULD NOT BE DETERMINED IF THIS CONDITION WAS PRE-EXISTING, BUT APPEARS TO BE THE MOST LIKELY CAUSE WITH THE AVAILABLE INFORMATION. NOTE - THE AIRCRAFT WAS SUBSTANTIALLY DAMAGED IN THE RECOVERY PROCESS, AS IT WAS DROPPED TWICE.