If you do decide to say no, then do it in a way that does not turn her off.
I have a non-frum relative, who was considering to become frum. When she was looking to rent from and live with a frum family with young kids, she was told “NO” in such a way that she was turned off. She is getting married soon (to a Jew), but I can’t go because the wedding is not kosher (food and aidim).