ACTIVATION LOCK

Is the phone locked to an iCloud account and/or locked to AT&T? If it is locked to an iCloud account then your friend will need to remove it from his iCloud account. AT&T has nothing to do with Apple's iCloud account.

If the iPhone is locked to AT&T, and meets ALL of the unlocking requirements, then use the unlock portal to unlock it from AT&T.

If my friend gave me an iPhone as a gift and he forgot his Apple ID password

. How do I get a receipt of proof of purchase so apple can UN ACTIVATION LOCK MY DEVICE.

You don't.  Only the original purchaser can request information on previous purchases on his account. If he purchased in an authorized retail store he must ask in that authorized retail store. If it was purchased from a corporate store or at att.com online he can request by calling AT&T.

If your friend was the original purchaser, they have to get the receipt.  AT&T isn’t going to give a proof of purchase to just anyone who happens to have possession of the phone at the moment.

Thanks for the solution, you made my day. If I want to know more, I will message you.