If you just ignore it, then it will probably just go away. But you don’t want to sit around waiting for that to happen, and dreading what the mail will bring every day.
So what I would do is… respond.
Write a draft of what you’ve told us. Be specific about the times and events and what occurred. (I would not mention your friend; you don’t need to throw her under the bus.) Be completely factual. Don’t be emotional. Say what your friend did if you want, and say what you knew and didn’t know, and when and how you found things out. Say what store employees told you, and what the police said.
Then take that account (which you should re-write a few times so that it is perfectly clear, complete, and accurate) and take that, plus the correspondence you’ve received from the store, and visit a storefront lawyer (or Legal Aid) and ask for a short free consultation. (Any lawyer who is interested in potential future business with you can spare a half-hour, even if he normally doesn’t do free consults, to read over what you will have to say and what the store has said.)
He may agree to write a letter to the store to present your side of things for a very nominal fee, and once the store sees that you have a lawyer and you aren’t simply rolling over and afraid of them, then they will probably drop the thing, because there isn’t that much they can sue you for, is there? If you’re an anonymous heiress and just haven’t told us that, then all bets are off, but I’m guessing that’s not the case.
Don’t just “wait to see what happens”. Make it happen your way.