This. firstly, If she wants to take your zed over her economic diesel, then she can darn well pay the petrol.
secondly, if her car has just one battle scar, thats one too many. someone who has car park bumps is not someone you want driving your car. especially since the zed will be a fair bit longer and wider, just increases the chance of a car park accident.
In addition, whats the state of her car like? Is it filled to the rafters with empty coke cans, tissues, spent parking tickets and the like? if so, then you've got to expect the zed will come back in a similar state.
And finally, whats her attitude to car servicing? if she doesnt service hers, it doesnt fill me with confidence she'll be mechanically sympathetic with yours.
If she wants to drive your car, i'd let her drive it along some country lanes, with you in the car to watch over her. i think if someone is let off the leash in a 280bhp rwd sports coupe they might get a bit overexcited.
alternatively, if she likes fast cars, then get her a trackday or a car control day
I have to say.... I totally disagree with almost everything you said