If your issue is just that every time she uses it, it changes the default back to IE, you can just open up IE and when it asks if you want to make it default, uncheck the box that says to always perform the check, then click No.
If your issues are that you are worried about security while she uses IE, you will have to do what the other people are saying. But I personally don't worry with all of the security garbage that people are saying about IE (even though I do use FF).
If neither of those are your issue, maybe you should just be happy using FF and let her use what she wants.