Should point out that copyright is automatically applied to anything whether it is stated on the work or not (although he should put his name somewhere).

Otherwise, if it ever comes down to defending artwork that someone ripped off and made a ton of money on, your best protection (assuming work of computer origin) is to keep a version of the piece that is not distributed and could not be recreated by anyone else, for example a higher resolution master, or perhaps if it was done as multiple layers the layered version.