I remember this one. The problem is the statement and his design is that it also changed his sales funnel. There was too much of a change to tell exactly where things fell apart or why specifically sales did not increase. Was it the initial offer? The middle? The actual offering? ect ect ...
I agree with this assessment that new/better designs don't always increase sales, though I believe (should he ever choose to pursue it more, though I don't think he will) he'll need to do a lot of split testing on the new design to optimize it like he did on his old design.
Plus, with the new design comes an increased brand image/presence which will help him in the long run.