Depends how traditional she is.
Obviously, if you're not Jewish, she isn't worried about your lack of kosher pots and pans.
You could buy a box of matzah, and have it there for her to put anything she likes on it. Fruit is good. Many vegetables. No cakes, no cookies, no breads of any other sort. Ice cream is usually good. There are some that are marked. You could go to the Kosher Aisle of the grocery store and find some prepared foods, there's usually a special Passover display, too...there will be snacks and packages there.
Coffee and tea are usually good (again, it depends how traditional she is...if she keeps more kosher, you should check the package and make sure it's Pesadic. Yuban and Maxwell House are.
My faves are Manischewitz macaroons, or just matzah with cheese melted in the toaster oven...again, that matters if she won't eat something that has been cooked in your kitchen.