Go and take some photos of all the pavers out there from various angles. And look over old property inspection reports to see if anything was mentioned.
The tenant not knowing her may not mean much, she could have come to a BBQ with a friend of theirs of something similar.
When did the alleged incident occur? As Marg mentions there are limitation periods when someone must bring a claim - and what state is the property in?
fair enough, many have parties where they don't know who their guests are. but surely you would remember someone at a party being badly enough injured to sue?