Why did apple filling in the pie turn to mush

Gluten is what makes doughs chewy, and gluten is associated with high protein flour, so you should try and get low protein ("soft") flour. Also, the colder your fat (i.e., butter) the flakier and crispier your crust will be. You also may want to substitute some of the butter for lard or shortening; the lack of water in the lard will also help the crust become crispy and avoid gluten formation associated with hydration of the flour. Note that you cannot substitute lard/shortening for butter in an equal ratio; you should try and find a recipe that specifically calls for it to get the ratio correct. As rumtscho noted, you can also blind bake the crust to start its cooking process (but as ElendilTheTall warns, you may not be able to call it a "pie" after that!). Finally, it may be the case that your pie filling is too moist and therefore makes the interior of the crust very soggy. I've found that almost all good apple pie recipes call for pre-cooking the apples to remove a lot of their moisture before putting them in the crust.

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!