Ford increased the GCWR for the F53 from 26 to 30K lbs. for the chassis Tiffin used to build the coach on so the drive train and brakes should be OK. Unfortunately the hitch Tiffin uses is only rated for 5K lbs and heading out to "Hitches Are Us" and bolting on a 10K lbs. receiver isn't going to solve his problem either.
The long frame beyond the rear axle and possible frame extension most builders add is of concern when tow weights climb beyond the 5K limit. Brakes on the trailer and slow smooth acceleration will help limit some of the stress on the hitch and frame but if the trailer ever starts swaying he could get into trouble quickly. He is risking twisting the rear frame rails, breaking the welds for the extensions or even pulling the hitch loose from the frame. All not good situations to be in when miles away from home. Your friend might be better served finding something lighter to pull behind the coach.