Was the mail sent from the insurance company itself, or from the place where he orders his medicine?  (Just curious, you said 'the insurance company sent it.')   
If the insurance company sent it, given that her daughter is being carried by your husband's insurance and EVERYTHING from them will have his name on it, then it's not surprising that she opened it and I think she was probably within her right to do so.  If it wasn't from them, but was from a company where he ordered the medication, then that may be different.   
In any event, I think the law applies more to stealing other people's mail, or opening mail that was misdelivered.  I'm just thinking that the law wouldn't really apply in the same manner given that they were married(?) and he has things that will inavoidably come to her mailbox.     |