Your friend is correct, if the body is not jacked sufficiently high enough. The shock, with the piston extended, is rather tall and it won't extricate itself from inside the spring if the vehicle is not fairly high in the air.

If you don't have it sufficiently high, you can jack up the trailing arm and compress the spring to give you some additional clearance to remove the shock. However, it's safer to simply raise the rear high enough and block the body before you do the job.