If she likes Patricia Cornwell, I'll assume she likes mysteries and crime novels. Hrm. There have been several mysteries available for free recently, but I think they have reverted. The only one I am sure is still available for free is The Score by Richard Stark. My advice would be to keep an eye of this forum for other freebies. Also, if she has a library card she might check if the library have e-books.
Also, check the e-book section on this forum. There are plenty of Public Domain e-books available in the Mystery and crime section
I hope this helped.