Hack into someone's messages?

Let's say you and your friend have the exact same smartphone. It is not an iPhone. How would you go about hacking into it from your phone into their phone without them knowing?

You don't.

Now if you get your hands on their phone, there are all sorts of apps you can use to forward their text, email, IMs to your phone, but you can look those up for yourself. But hacking, it ain't happening. Oh and using an app to do what you think you can do, would not be hacking anyway.