It's very generous of you to let her play in your world. My impulse, like your first one, would have been to say "no."

But you've put acceptable parameters around it, and it sounds like she respects not just your writing but your right to your work.

The way I've seen it, you've "paid it forward." We borrow from original works for our fanfic, and now you've had a chance to let someone borrow from you. Bless you for being gracious.