What happens after Madam Forestier realizes that the necklace she received from Madam Loisel was a real diamond necklace?

This happens at the very end of the story. Let's take a look:

Madame Forestier had halted.
"You say you bought a diamond necklace to replace mine?"
"Yes. You hadn't noticed it? They were very much alike."
And she smiled in proud and innocent happiness.
Madame Forestier, deeply moved, took her two hands.
"Oh, my poor Mathilde! But mine was imitation. It was worth at the very most five hundred francs!..."

As you can see, after ten years of hard work paying for the replacement necklace, Madam Loisel mentioned it to her former friend, Madam Forestier, saying how she was finally finished paying off the replacement for the one that Madam Loisel had lost all those years ago.

But Madam Forestier is shocked and "moved." She reveals that her own original necklace, the one that had been lost, wasn't even made of real diamonds! It was a fake—probably a very nice-looking fake, since Madam Loisel assumed it was real and enjoyed wearing it at the fancy party, but a fake nonetheless.

What this means is that Madam Loisel and her husband have been wasting the past ten years of their lives with worry and obsessive working to replace the necklace. If she had just been honest with her friend and told her right away that the necklace was lost, then Madam Forestier could have said, "Oh, don't worry about it. It was just cheap costume jewelry." (As she explains, it was only worth 500 francs at the maximum, not the 36,000 francs that it took to purchase a genuine replacement.) Instead of being honest, though, Madam Loisel tried to hide her mistake, and the guilt and burden of the lost necklace rankled her for a decade.