Downshifting into lower range is not damaging unless ur in speed higher than the maximum that is allowed by the lower range which ur shifting into....( from D to 2 )
u dont have to make sure to come to a stop to change into lower range...., just make sure u werent exceeding the maximum speed allowable for that lower range....
and also shifting into N while driving is not harmful, but shifting to P while driving is harmful...
shifting from N to D while driving and then coming back into D can be harmful....
shifting to D from N at high engine rpm can damage torque tube....
to check torque tube, press the brake fully down, and shift into D and press the accelerator to the floor as well.....The rpm shudnt come anywhere the rev limiter, it shud be around 3k or so, depending on car.......
And the driver of ur friend might be driving an auto for the first time....