Optimal shape to cut apples for apple pie
I've eaten plenty of apple pies with crescent slices. Nothing wrong with it at all. If you don't have any special tools, it's probably the best way to get slices that are the full length of the apple. It's probably what I'd do if I made a pie tonight.

A lot of people probably slice differently, though. In particular, if you have a corer (which doesn't also slice) you can peel, core, and halve the apple, then cut thin slices perpendicular to where the core was. It's easier to get thin slices this way, since you're making parallel vertical cuts. Of course, it's a pain to core a whole or half apple without a special tool. 

Perhaps some people out there, especially if using very large apples, peel, quarter, core, and then slice, getting quarter-circle slices. Some people prefer small pieces; they might use a corer/slicer to get get 8 wedges, then slice those further.

People who make a lot of apple pies might have a device like this peeler/corer/slicer. It does all three simultaneously, rotating the apple to peel it, pulling it across the corer, and slicing in a spiral as the apple rotates and moves horizontally. Theoretically you end up with a peeled, cored apple that's thinly spiral-sliced the whole way through. At that point you can just cut it straight down the middle, and end up with half-circle slices.

I expect your friend was simply used to one of those methods. But any kind of apple pie you can think of is being made out there. Slice them how you like!