Were they parcels or letters?

If parcels, check the tracking and if it shows delivered, point that out to her and advise her to enquire at her PO as to how the item can be marked deivered when she hasn't received it.  If she is not genuine, that will probably be the last you hear.  If they were letters, there is not a lot you can do but refund, as there is no way to prove delivery.  That's the drawback of using letter post to save your customers money, while most appreciate it, there are some who will take advantage of it.