Can't connect iphone 4 to pc, asks for unknown apple ID
I have a difficult problem with my iphone 4. I live in Mexico and a friend of mine who lives in the US, sent me his old Verizon iphone 4 as a gift, since he was getting a new one, it was unblocked and ready to use with a new company.
So I went to a local store to be able to use it, the company is Unefon, they told me they were the only ones who activated such phones (it's an iphone 4s -I think, it dowsn't work with chip). At that time, it had ios6 installed.
They told me it would be available to use within a month, so I left the phone there... One month later, I came back and they told me they didn't get some code they needed, they ask me to come back in a month and when I got home I turn on the iphone, it turn out that it was blocked, as if they tried to activate it several times with a wrong password, I couldn't even use it no more!
I looked for a tutorial on internet and I found everywere that the only way to unblock it was to update ios6 to ios7, so I did that, but when I was almost finishing, it asked a password for an unknown apple account.
I ask my friend about it and he gave me his, but the email the iphone showed wasn't his, and we didn't know it.
I went back to the store and told the guy to fix it, he unblocked the phone, but then, when I went back home I tried to connect it to my computer and itunes is still asking for this apple ID...
I've looked everywere for answers and I couldn't find where to start. I even ask my friend if he stole this phone or something similar, he swears it isn't stolen or bought from a thief, it's his. Any ideas on how to fix this problem??