Hi,
Sometimes it is better to just give a refund, particularly if you do not have track and trace and have no way of knowing where the package was.
Some time ago a customer whose package was, according to USPS, at the NY sorting facility, filed a case against my shop just 20 days after I shipped the package. I had no option but to refund her, even when the package was probably just delayed.
As a seller you have very little protection. I would offer her to send a new item or refund her and forget about it.