A persons name may not be unique. Are there any others in the world with that name? It is difficult to get a domain from someone without a trademark. Since I would doubt he trademarked his name he would have no recourse there.
If he has an agreement with the art gallery similar to what most mainstream sponsors have then it would specify that "like domain names" and "bidding on keywords" are not permitted. Again I doubt that your friend has this agreement.
Without the agreement or trademark there is not much your friend can do but negotiate with them on a fair settlement for the domain.
If they are not will to give it up then he could just break the reseller agreement and it would become useless to them as they would not have the prints that go with the domain for sale.
Take this however you want as it is only my
.