Does your 'good player' buddy have a loose belly by any chance ... because that also causes the hips to 'block' in the backswing and downswing too?
Another condition would be the alignment of the knees .. bow-legged or knock-kneed .. because that will defeat hip rotation too?
Then there is the condition of his hip joints too. Did he suffer from Juvenile Rheumatoid Arthritis in his childhood, because that can affect the hip joint mobility in adulthood. Of course, he could also be suffering from hip joint arthritis due to overuse trauma. Did he always swing without hip joint flexibility??
Also, what is his height, weight and shoe size .. because it could just be a balance problem in his inner ears!!!
Also, you should classify his body genotype and optimal golfswing according to LAWs of the Golfswing - Adams, etal.