Must I download Xcode with the same Apple ID that I'm submitting with? I have an apple account and I have downloaded Xcode on my Mac. If I download Xcode on my friend's Mac because he doesn't have an account yet and he would be on the iOS program later, would he be able to upload his app (saying he downloaded Xcode with my account and he now has an apple account)? Should he download Xcode again (with his account)?
Your friend will be fine. The account used to download Xcode is in no way connected to your iTunes connect account, which is what is used to upload your app later. Xcode prompts for login information where appropriate, so your friend has nothing to worry about. There's no need to download it again later.