paint correction can be done, for the most part but it can always be a case by case basis. You can do a test spot and based on those results you can either duplicate them on the entire car or do another test spot in a different section of a car to dial down your method. Measuring the paint is always a good practice
As far as orange peel goes, it can be removed without repainting the car, if you have enough cc to start out with, but then you'd be left with less CC on the car. Which if the owner wants it removed and does mind living with a minimal amount of CC and he knows the drawbacks of this procedure, then in the end it is up to him.
Removing orange peel on a OEM paint job can be an advanced job. I'd be careful in tackling it if one does not have the experience and know how on how to do this.
A different option could be to get the car resprayed with additional layers of CC to have enough CC to level out the orange peel and leave the CC with a mirror finish. That however is a whole different ball game.
^ just my opinion and also just trying to help.
Good luck

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