I don't know of any good free keyloggers, but I'd actually go in a bit of a different direction. Instead of finding out what was typed in later, I'd install child filtering software, which will block objectionable content. Naomi is a free option, but it's not customizable or advanced in any way. Another option is Content Protect, which is the best in its class, completely customizable, nearly impossible to disable, and includes full activity logging capabilities. It's not free, but it does have a 2-week trial, which should be fine for her visit. Based on the results you can make a decision as to what limitations and/or punishments will be set.

Hope this helps,