I'm not sure what you mean by unclaimed--is that the reason that it was RTS, not picked up from the post office ( as opposed to "moved", "addressee unknown", etc)?
Unless she told you before you shipped/tells you now that it is still the correct address, then you can't assume that it is.
I'd email her and let her know you got the package back RTS and the address you shipped it to and that you need to hear back from her.
If she confirms that it is the correct address, I'd offer to re-mail or refund--it'd be an error on the post office's part and you might get the money back/free re-shipping from the PO, but it's not the buyer's fault so it falls on you to take care of it.
If she says it is not the correct address, I'd offer to re-ship once she pays for the new shipping or refund (item price only, not shipping).