It's nice to see the world is still full of good people.
You can return the card to Frontier Airlines, Attn: Early Returns, P.O. Box 17304, Denver, CO 80217.
Also, if you could put a little note in the envelope stating why it is being returned, that would be helpful too.
Thanks for taking the extra step to try and reunite the card with its owner. I'm sure she will be very happy.