We are in a bit of a conundrum and I'm wondering if any of you have any suggestions.

We are due in March and although we have a lot of friends who are willing to watch our son at moments notice all of them work so we are gonna need a grandparent to be able to hop a flight and swoop in as soon as possible. I'm wondering what options we might have for this. My MIL lives on Denver a 2.5 hour flight away and it's a hub so there should be a lot of flights. Is there a way that she can buy a ticket that she can use anytime?