That’s a very good question. I do The Necklace almost year with my underclassmen. They get a kick out of the ironic ending, as Madame Loisel finds out that the necklace she lost was a cheap piece of jewelry and should have cost very little to replace. Instead, the Loisels mortgaged their entire future to give Madame Forestier a necklace that they thought was comparable to the one they lost.
The problem, of course, revolves around the fact that they never told Madame Forestier about it, so they never knew that they were wasting all that money.
Invariably, when we finish the story, a student will suggest an alternate ending without being prompted. Often, they suggest that Madame Forestier should be made to reimburse the Loisels for the money they spent on the replacement necklace. Another ending that is suggested is that Forestier simply give the Loisels the necklace back.
In line with the above answer, it seems only just and reasonable that Madame Forestier should give the Loisels the replacement necklace and that they would sell it in order to have a little security and enjoyment for the rest of their lives. I would assume that Madame Forestier would feel the same way--although in Maupassant's world you never know what upper-class people are going to do. I don't know if the Loisel's would have any legal recourse--but they would probably threaten to sue if the woman refused to give up the necklace or pay them its market value.
The story has a shocking ending, but after the shock wears off, the reader begins to wonder what actually happened after Madame Forestier told Madame Loisel that the necklace she had let her borrow was made of fake jewels. This shows the genius of Guy De Maupassant. He has us convinced that these are real people and that this really happened, so we are left wondering about the final outcome.
Maybe some creative person should write a short story beginning precisely with the revelation that the jewels were fakes. In fact, an English teacher might assign an entire class to writing such a story. On the other hand, what might be more important than the cash value of the necklace could be the psychological reaction of Mme. Loisel and her husband to the revelation. How would it affect their relationship? Maupassant's story slightly resembles O. Henry's "The Gift of the Maji."