How to tell if an iPhone is hacked and how to prevent it? A friend of mine said he thinks his iPhone has been hacked. How do you find out if it has been and more importantly how do you stop it? 

He called me up because his girlfriend says things during fights that he mentioned in text messages. He said she let it slip she ‘remoted’ in to his phone.

I think I know one way this could have happened. If she backed up his phone to her Macbook then iMessage on her computer might be receiving his messages. But that would only work if she has a Mac unless there is an iMessage for Windows.
I don't really know why people always think their device is being hacked after something weird is happening in their eyes.

If she made a backup on a computer, and someone put that version back - they have access to the complete iPhone after restoring it on another device. Can this be the case?

If not, he said things during fights on iMessage? Maybe he just said it and regret it afterwards claiming he didn't send it. I don't see why anyone would add extra text to a fight and not use it for something else (if they really have bad intentions.)

Backing up an iPhone to a Mac does not turn on iMessage by default unless the user activates this him/herself. It is impossible to get active just because of a backup.

Solution
Change the Apple ID password and all default Apple services will become useless because it will prompt for the new password.