You need an attorney to try to prove your case. It will be difficult. Quite frankly, even if surveillance tapes show your friend putting items in your purse, that does not prove that you weren't part of the shoplifting team. And, since security does not routinely check every woman's purse as she exits the store, the fact that they stopped you implies that they may had seen activity, either directly or on the surveillance cameras, that made them suspicious.