iPhone 5 activation lock

My brother gave me his iPhone 5 because my phone is broken.

I reset everything.

But I encounter three problems while I was trying to set up the phone:

First problem is that it has some kind of activation lock.

Second problem is he bought the phone for his (now) ex girlfriend along with other things and she had to give him his phone back after they broke up.

Third problem is the account under her name.

I'm guessing she's doing this for revenge or something??

My brother has the box but i can't find where he put the receipt.

Is there any kind of way to get this lock thing off?

If and when I find the receipt can I have apple to take the activation out?

What can I do?