In "The Necklace" by Guy de Maupassant why do you think Madame Forestier did not give real jewelry to Mathilde?
I have seen a few different translations of "The Necklace" in my time, but none of them has ever suggested that Madame Forestier deliberately chose to give Madame Loisel a fake necklace. A close reading of the versions I have seen show Madame Loisel making her own choice from several pieces offered. There is nothing in the text that could be interpreted to show that Madame Forestier offered her only fake jewelry to choose from, much less why she would do so. And the ending of the story suggests that Forestier had no idea that Loisel had believed the necklace was real diamonds.
Generally, as we make meaning from a text, speculation about a character's motives for his or her actions can offer us insight into a story or into ourselves. But when we are reading into a text a deliberate choice that is not supported by the text, I think we are drifting too far.
Having said that, I would guess that if Madame Forestier did deliberately offer only fake jewelry to Madame Loisel, her motive could have been as simple as not wanting to lend anyone, even a good friend, a very valuable piece of jewelry. It is also entirely possible that Forestier did not own any "real" jewelry in the first place.
In one of the Sherlock Holmes stories by Arthur Conan Doyle a blackmailer tells Holmes that the woman the detective is representing could easily raise the cash he is demanding for some incriminating letters by "turning her diamonds into paste." This may have been a common practice. A married woman like Mme. Forestier would have received most of her jewels from her husband. It seems possible that she might have wanted to raise some cash in the past and did so by having a real diamond necklace duplicated with fake jewels, intentionally deceiving her husband but probably not wearing the fake necklace very often. Naturally she would not want to admit to Mathilde Loisel that the necklace her friend wanted to borrow was a fake. And Mme. Forestier was probably glad to be able to lend a cheap article rather than one that was more valuable. In fact, it seems likely that Mme. Forestier would have been vastly relieved if Mathilde had only confessed that she had lost the necklace. That way Mme. Forestier could tell her husband the necklace was irretrievably lost and not have to worry about his discovering that it was not the genuine one he had originally given her.