Another thing to consider is calling DL reservations. Explain that she really needs to be in F due to injury or not travel. Ask what they can do. They have more options than Skybonus agents would even know about. They also can waive change fees, etc that SB agents can't.
Lastly, have you pulled up the reservation online or in app to see what the upgrade offer is? Depending on route/day this can be very cheap and less than upgrading to what a current F fare is.