What is the reason she is missing the first flight? Is it she just doesn't want to go, or is she sick, has a serious/conflicting commitment, etc.?

DL is much more reasonable than AA in working with customers who have legitimate issues come up and can not make a particular flight. It's all at the agent's discretion, though.

Best to call and let them know what is happening, ask them to separate out this one person from the rest in the reservation, and see how they can work with you. They likely won't just cancel the outbound and let you keep the return, but they could very well waive the change fee (or reduce it by 50%) and let you refare the return at today's price. That would likely be a fair compromise, again depending on the reason for the change.