For a fan that is desperate to get a copy of the necklace it will seem that the best thing to do will be to get a copy made. There will be a lot of independent jewelers who will be able to make a copy. There is a plus side to this and that is the fact that the replica can be made exactly as the owner wants it. If the cost was going to be prohibitive cheaper materials could be used or if it did not sit quite how the owner wanted it to then it could be done a different length. Whatever changes are possible ands as this is not bought readymade it will cut out the cost of alterations.
By trying an established jeweler they may recognize the design and be able to say where the original came from as this would stop them having to worry about copying someone else’s design. It may be that the person who makes the copy will insist on a few small changes just to make sure they stay on the right side of the law.