iPhone 4 boot loop, no DFU possible please excuse, I am new to iPhone, as I repair it for a friend.
It's an 4S from USA that some other friend of him wanted to unlock for germany.  

So I don't know what he did exactly, but:
When I restart by holding home+power, it restarts like expected. The apple appears for 2 minutes, then the (respring?) turns 180 degree and then stops.
There's also no way to get it into DFU mode, it boots no matter if I hold the home button.
Also when I hold vol+, it won't boot. 

I'm out of ideas to get this to boot.
I could fix it with some luck now.
I didn't restart by holding power+home for 7 seconds.  

I pressed them for about 5 seconds, until the screen turned off.
Then I held the home button, and connected it to a pc withouth iTunes installed. (This didn't work with a pc with iTunes!)
Now it asked me to connect to iTunes, so I connected it to another pc having iTunes installed.
After iTunes finished restoring, the phone worked perfect again.