There are free programs that do a simple write-over of empty space with FF. This is enough to thwart common data recovery tools. If I remember, I'll see what's on my work computer. When I left my old job, I scrubbed my hard drives. Those same tools will do a much better scrub where they write over the free space multiple times with random data if you think someone will do a fancy recovery of data.
Beware, if you used this machine for accessing password protected accounts (banking, email, ...), Windows likes to store these passwords. I have seen a tool in action pull out all sorts of fun password data. Why Windows does this is baffling. If this is so, clear off the old drive and reinstall the OS and programs she needs. Windows is really a dangerous OS when it comes to security.
BTW John, there are everyday tools to bring back deleted files, including freeware on Sourceforge. I've done this many times to get back accidentally deleted files on crashed drives and servers.