What's happening to my iPhone and his?

So I set up my own account on iTunes synced it to my laptop, and my dads friend ask me of I could do his, that was fine set it up in HIS own account. So mine and his are different BUT he keeps some of my texts and gets all the new apps I download and music.
He said to my dad he doesn't know half the stuff going on with his phone he just goes with the flow.

But how come he's getting some of my messages? That's weird I don't want him knowing what I'm sending -.

Messages could be from a messaging app (not the stock messaging app). And the apps that are downloaded to his phone is weird. The only way apps go on to his phone is somehow when he goes to his itunes, its linked to your account and he gets the apps from there. Make sure your accounts are designated to each phone and itunes. This is the best advice I can give you.