How to transfer music from a iphone to another iphone 4s

Ok, so guys my friend left his iphone 4s with me and its full with music and I would like to add those songs to my itunes.
How can I transfer from his iphone 4s to my computer so I can than transfer it to my iphone 4s?

Isn't there something called an iCloud? Not sure if that is what you're talking about, but I think it's just like a backup of everything on your phone.

Ill look it up more when I'm at my computer but I know it's very complicated for a stupid reason. If I remember right when my old roommate and I tried you could only play music from one persons account. Must be something with the drm

Check this out, when the music is on your pc you can just put it on your iphone the usual way.

I haven't tried this but it was just something i found on google

You can't, unless you know his Apple ID. If you try to sync it to your computer without his Apple ID, it will wipe the content on the phone.

Source: I've done it before.