right of publicity," a maxim that oversimplifies a complex issue. The right of publicity allows every person to control the use of their name or persona for commercial purposes -- typically on a product (like our Tiger Woods golf ball). However, celebrities cannot control so-called informational or "communicative" (first amendment) uses -- for example, articles, books, or documentaries.
What about the gray areas? Things get tricky at the intersection of communicative and commercial uses -- such as limited edition art prints. In that situation, a ten-year old Ohio ruling favors your friend. An artist painted Tiger Woods at the Masters Tournament and later sold more than 5000 prints of the image superimposed with other great golfers. (Here are some details on the artwork). A federal court of appeals ruled that the sale did not violate Tiger's right of publicity. A related ruling regarding paintings of the University of Alabama's football team -- though it dealt more with trademarks than the right of publicity -- also was in favor of the artist. Both of these cases took years and a great deal of attorney time and fees to decide. So, although the law may be on the side of your friend, that doesn't mean he can't be dragged into court to defend his rights.
No copyright infringement. Also, we're assuming that the prints being sold are made from your artist friend's original paintings. If the paintings are by another artist, or if they are based on a photographer's original photos, then your friend could be hassled for claims of copyright infringement by the original artist or photographer respectively.