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.