My instructor did the same thing. I taxied back to the flight line and made the turn up to the the double lines and stopped. I told him to hit the kill switch on main and he did just what your instructor did. Luckly, mine ran into the netting and no damage, but the nerves were baddly frayed. I'll bet this happens a lot.

I found one other issue with the buddy box setup. The trainer box wouldn't give full throw on the throttle. If I trimmed up the throttle on the buddy, I couldn't get it down to an idle for landing. If I trimmed it for idle, I couldn't get much above 6000 at full throttle. Everything else tracked with 1 for 1 movement between the boxes. The master is my Futaba 9CAP supper and the buddy was an older futaba four channel.

Don