If I were your friend I would try two things:
. If it is possible he can patiently sand the fish with a very fine grit sand paper or some spongy sanding equipment like 3M ultra fine sanding sponge. Both of which can be used wet or dry.
. If the type of wood is possible to soak in warm water without bending or deforming afterwards, water based paint can be brushed off gently if the fish can be soaked in hot water for some while.
For fine details like scale marks etc. he may need a pointed metal instrument to remove the paint inside the grits without damaging the marks, paitence is needed I suppose.
When it comes to painting it again he may use diluted paints. Here is a link for an old thread about painting wooden bracelets by me: http://www.wetcanvas.com/forums/showthread.php?t=372122
If you search for some of my older threads you'll find some examples of painting wooden boxes, Spanish guitars, etc. which can perhaps help you to decide how to paint wood very thin.