You have a reasonably good chance, especially if you get your husband's mother to come to court and testify not only that it is her car and she left the bottle in there, so that 1) that the meds are hers, but also 2) she hasn't seen any unexplained drop in the number of meds (i.e. no reason to think anyone is taking any). You need her testimony: the legal system requires witness testimony for evidence, and does not let you simply put in unauthenticated documents, like the prescription.