just download a second browser for her to use.
for example I am on a mac...I use Firefox and anyone one else who comes near my computer uses safari. I can breathe easier that way. I mean, not that I do any sort of questionable internet surfing but I am just very private
even better then a different browser is to set up a user account/log in for her with limited access to files and applications. I have a guest account set up on my machine. When my bf (or anyone else for that matter) comes over I reboot into that account for the duration of his stay