first end the auction - there should be a drop down menu on the side? or follow this link
http://pages.ebay.com.au/help/sell/questions/endlist-now.html
then create a special listing as a BIN for the buyer, give her the new item no - don't put too many details because you don't want someone else to snap it up
you can have as a title SPECIAL LISTING FOR XXX(buyer id)