I do classical, it doesn't matter just do what feels most comfortable. One advantage to being used to playing classical is it makes playing flying vees much easier.
Quote by fail
\/ <-- my care vagina is empty
Play however it's comfy! I tend to have it at an angle when I'm slouched on a sofa or bed playing, fairly level if I sit up straight, and if I'm standing, I do a bit of both.
Quote by Kensai

Awesome guy right here
Your friend is trying to make you change your posture purely for the sake of conforming. Don't do it. Classical posture actually has a lot of advantages. For one, it puts your hand at a better angle for big stretches. Also, with the guitar held horizontally, your elbow will hit your side once you get up into the higher frets and you're forced to use it as a pivot point. With classical posture, your arm is free no matter where you are on the fretboard.

Holding the guitar horizontally just looks a lot better. But if your playing is good, it's a non-issue.