Flight Instructor reported the student; in the instructor's charge; taxied into the running propeller of another aircraft. There was damage to the student's aircraft wingtip and no visible damage to the other aircraft. The instructor was not in the student's aircraft at the time.

Narrative

I am a Flight Instructor and was giving my student the last flight lesson before his Private Pilot checkride in ZZZ1 area. On the way back to ZZZ; we landed; and on the way to the park; I had to urgently go to the restroom. I made my student idle the throttle for me and hop out to go to the restroom. He did idle the throttle right before the fuel stop corner; so I could disembark. I was confident enough that the student is capable of handling that aircraft without my assistance since his check ride was planned in just a couple days after the incident and since the student has close to 120 hours Total Time. I admit I should have been inside the aircraft until the lesson finished because I am pilot in command and I need to make sure. Unless it is a solo flight and students make mistakes easily. After I came out from the restroom; I went to the parking area and noticed something had happened. I wasn't in the cockpit at that moment for the incident; but for sum up what I heard from the people who were on each aircraft is the following:On the way to taxi to park; there is always an Aircraft Y parked kind of dangerous(or sometimes hard) for the other aircraft to avoid hitting the tail (Our Chief Flight Instructor; said several times to the owner or someone in charge of that to move the aircraft on the safe side but they never did)By the time our aircraft was trying to taxi to the parking area; the trainee didn't want to hit the tail of Aircraft Y so he taxied more cautiously and a little bit more to the left side as most pilots do to avoid any collision with that aircraft.Aircraft Y; operated by Company A; had just started the engine; got the ATIS; and was to taxi; checking the brakes for each side of control by moving a little forward. They were sticking out of the parking spot on the taxiway with the engine on.At the same time; my trainee was trying to get into the parking lot and both aircraft collided. Our wingtip was cut off by the running propeller of Aircraft Y. Only the wingtip seems to be damaged by an on site mechanic's inspection. No damage to the wing itself and also there was no visible damage on Aircraft Y after the collision.Recommendations / Suggestions / Comments: The student should have some ground lessons from the Chief Instructor regarding safety operations on ground.I will be having a meeting with our Chief Flight Instructor to prevent occurrences like this.If it's not a solo operation; I will be required to stay in the cockpit at all times for the safety reasons.Safer taxiing operation from the ramp; anytime we see something moving (aircraft; vehicle; person; and so on) we need to wait for them to get out of the possible danger zone. We need to avoid any unnecessary collisions.