Hi Andrey,
This may sound like a dumb question, but are you sure your friend remembered to remove the protective plastic over the hole for the proximity sensor? You can see what I'm referring to in this picture from the iFixit Store.
[image|2794268]
It's such a basic issue, but believe me, I've dealt with that exact same problem with other users here on iFixit and in my own repairs.
I'd love for that to be your problem, since the fix is so quick and simple; just remove the plastic and you're good to go.