Stepping back, the strength of feeling of the various posters is quite interesting. If I have a strong reaction to something involving someone else, that strong reaction sheds a light on some issue I have more than anything else.
There are many factors not yet known which would affect how strong or weak a case the artist would have legally. What if the artist had explicitly told the curator that $3,000 would be spent on prepping a display specific to the coffee shop setting. Then that actual knowledge could be used as a basis for seeking $3,000 in damages. Absent a way of showing actual knowledge of the $3,000 having been spent and absent proving that it was indeed spent specifically for that display, the artist would be limited to whatever could be proved to have been the amount of income lost from the time the display was taken down up through the time it should have been taken down (i.e. the 6-week mark). Such proof would usually be quite difficult. Parenthetically, this assumes that one could prove the curator was the agent, real or ostensible, of the coffee house owner. Lastly, one would want to know if the contract have an attorney-fee clause. In the real world, that might be the deciding question underneath all questions as to the merits of the case.
If, out of idle curiosity, one were wondering about something other than breach of contract, off the top of my head, and based on the very limited information, I'm not coming up with any legitimate extra-contractual claims.
I mention all this boring legal stuff because it is independent of aesthetics and taste and is, for better or worse, all that really matters in the real world sense of what is arguably right or wrong. All the other considerations are a matter of opinion, and that just gets us arguing about the number of angels that fit on the head of a pin.