Welcome to A2K trickysitu! Interesting dilemma, and I too can see the merits of every position posted.
Personally, I’ve no choice but to agree with the "own it" crowd and would waste no time informing her that she had two choices:
1. You return this item and apologize to these people.
2. I'll return this item and apologize to these people.
Then I'd paint a pretty clear picture of the former being the infinitely more forgivable option in the eyes of the victims, as well as the court's should the victims choose to pursue justice.
I doubt very many people would be spiteful enough to call the police back into it, considering they would by now in effect be punishing her for doing the right thing, as well as the insurance angle that really is pretty likely and perhaps even the reason for the police’s initial involvement. Even if they did, I would be surprised if the State's attorney wouldn't be happy to offer a deferred prosecution agreement (providing this is a first offense)... in which she would only need to satisfy the State's conditions (Like counseling, fines, costs, community service, etc.) and keep her nose clean for a length of time before the matter was erased forever from the books... though as Panzade indicated; the lesson would remain learned for life. And it's a damn good lesson. One well worth the risks, in my opinion.
That being said, I don't know her history, the victim's personality, the value of the item or how tough the authorities are in your area. There are certainly risks... but that's the reason she shouldn't have done it... and facing same is the reason she may never again.
Whatever you decide; be sure those people get their property back pronto, one way or another, or just by your writing here, you may very well be considered a party to the crime. Possession of stolen property becomes an issue at about the same time you become aware of it.
Best of luck, and please do us a favor and let us know how it plays out.