I've seen people fit chequer plate covers to the rear cross member. to hide the whole crossmember. If the tester can't see it he can't fail it!! But what your mate has done will be an instant fail. As soon as the tester sees it he'll fail it, and he will see it. As the rear crossmember is one of the 1st places he'll look for corrosion.
Problem is if the rear crossmember has gone on the outside, then you can almost guarantee that the rest of it will be rotten and the testers little hammer will find it..