It is important for her to keep fuel in the car so that she doesn't look for fuel late at night. Hmmm, maybe it would be best if a 16 year old girl isn't out late at night anyway. Even on Friday and Saturday nights my 16 year old daughter's curfew is 11:00PM. She is sometimes ten minutes late, but she has done well with this, I am very proud of her.
The "hard to find diesel fuel" argument doesn't wash in my book. She should not get in the habit of waiting until the reserve light is on to look for fuel.
I agree that getting on the freeway, extra power is a safety situation, but it is more than offset by the fact that you're not putting the teenager in a car that they are tempted to drive fast.
I put my precious daughter in a slow diesel almost a year ago, and I still think it was the correct decision.