you can ask for recorded delivery yourself - it is only about 70p, just add that to the total.
the only way the royal mail will have a record of delivery is if it recorded - and therfore she will have a receipt - ask for it.
also check that her feedback score is not just her buying loads of stuff - it is her selling score that counts.
some buy loads of cheap items just to bulk up their scores
it si her responsibility to recover the item, which is something many sellers don't seem to understand.
when she posted the item, she opened up a deal with the royal mail, for them to get her item to you - until it is safely in your hands, the contract is still open and up to her to deal with.
she must refund you and claim against royal mail
the PO will not discuss this with the recipient