I hear what youre saying, but regardless how good your friend is at doing the prep work, PT isnt going to put their name on the line, or have the possibility of it coming back with issues over someones word. We had the same type of situation happen at the body shops alot, people that “prepped it” and just wanted it sprayed, come to find out that they we no where ready for paint. Its just extra insurance to do it your way, and make 100% sure that it is correct.