Perhaps you have been done! Mine is 17 years old and although it has lost its shiny finish, it is still white gold (silver) coloured and I do a physical job so it gets lots of wear.
Has he done any diy recently or in the past, or used any thinners while wearing it. As this will react with the white gold and turn it yellow. (i had the same problem) The easiest thing to do, is to buy a jewellery cloth from a jewellers about �5 (i got mine from Goldsmiths) and it will bring it up like new and the cloth lasts for years