You can’t have it both ways.
The story says she rented the car for two days and returned it on the day her contract obligated her to.
She fulfilled her part of the contract returning the car both on the day and in the manner she was instructed and indeed required to do.
It isn’t her fault Enterprise failed to secure the car after she fulfilled her obligation to return it. No court is going to hold her responsible when she did exactly what was required of her under the contract. If Enterprise wasn’t able to receive the car on a Sunday they shouldn’t write a contract requiring it returned on a Sunday.
That’s like renting a movie for 24 hours and then the next day finding out the business is closed for the day and they want to charge you for another day.
And this wasn’t “after hours”. It was on a Sunday when Enterprise required the car to be returned or face additional charges.
This is entirely Enterprise’s responsibility.
It’s not both ways. The customer understands that until Enterprise comes into possession of the vehicle, it is her responsibility. If a customer signs a contract to that effect, should that signature be considered meaningless? I am truly sorry for the customer that this happened, if I haven’t said so.
You are mistaken in thinking that Enterprise will pay for the Mustang. Every other Enterprise renter will pay for it, via increased costs.