So wait, let me see if I've got this straight. Mabel returns one car, immediately gets another (it sounds as though she didn't leave the lot and just switched cars) for no reason aside from her desire not to have to get gas. Then, when the clerks at the car rental place didn't jump through hoops so she could get her new car from them when there was nothing wrong with the old car, she came to you, someone thoroughly unconnected with the situation, to complain? Wow.
I would have said something like, "Well, Mabel, they're not used to people renting more than one car in a day. There's bound to be some hiccups. Now, I've got to get back to (whatever you were doing). Have a nice day!"
Of course, inside I'd be mentally asking her if the hassle of the car swap was worth the amount of money it would have taken her to get the car up to half a tank of gas, but of course I wouldn't actually SAY that out loud.