If your friend is talking about Busycon contrarium (or whatever they are
calling it these days), they gradually become lighter colored as they get
older and larger, and very large ones are almost always white.  Also, I don't
know why anyone would want to turn a Busycon white, but many shells can be
turned white, especially those that are light colored to begin with, like
Busycon, by baking them in an oven.  350 degrees F for a half hour or so will
destrot many pigments (beware when purchasing "albino" specimens!)
Regards,
Paul M.