S.M.
I don't know how old the girls are, so my answer is going to be a bit general.
First of all, you need to get the word "punishment" out of the picture. They're having relationship problems. What the one did by stealing was wrong, yes, but you also have to consider that the one who was stolen from may have done something rotten, too. There are TWO SIDES to this story, but you have only told one side. I think you need to be way more unbiased.
You shouldn't try to force the girls to be BFFs. The sister who gave the BFF necklace was saying something way beyond the gift of a mere necklace ~ tweens and teens take BFF very seriously, even if their BFFs change weekly.
So...I would not force the stealer to fetch back the BFF necklace because you can't enforce the concept of BFF. What I would do, however, is find out what happened to cause this serious breach of affection between your daughters and have them make amends TO EACH OTHER.
Unless the stealing daughter has a history of stealing things from sis and others, it sounds like this was a one-time theft, and it had a specific purpose ~ to send a message to her sister that "You're not my BFF anymore." It's OK for her to be angry at her sister, but it's not OK for her to steal. The BFF concept is so important to her, however, that it made her break the rules about stealing in order to make that statement to her sister.
I think that healing the sibling rivalry between the girls is WAY more important than the necklace (which cost what? a couple of $$?).
Fix the underlying problems between them, and then worry about the little things, like the silly necklace.
Peace,
S.