New 24-85 vs 16-85

The 24-85mm might be slightly sharper on the edges of the frame on a DX camera.  But I agree that it isn't as important as whether he will be happy with his zoom starting at 24mm instead of 16 which is a significant and very noticeable differnce.  Personally if he already owns the 16-85 my advice if he wants better IQ would be either the 35, 50, and 85 1.8 primes or the 17-55mm depending on how much he needs the convince of the zoom.