How to unlock a activation locked iPhone

Only by using the correct Apple ID that was used to lock.

Activation Lock is a theft deterrent feature with no bypass or workaround.

Without the owner's authorisation, no thief or anyone including you can re-use the device.

You can do nothing whatever. Your friend needs to follow the instructions here to retrieve the ID:

If you forgot your Apple ID - Apple Support

and to reset the password:

If you forgot your Apple ID password - Apple Support

Failing that, he (not you) should take the device, the original purchase documents, and some ID to an actual Apple Store (make an appointment) and ask if they can unlock it.

If he can't or won't do this you should insist he takes the phone back and refunds your money: I reiterate - there is absolutely nothing whatever you can do about it - Apple won't help you.

Turn off Activation Lock https://support.apple.com/en-us/HT201441 “If you erase your device without signing out of iCloud first, it might still be protected by Activation Lock. Whether you erased your own device or … [got an erased] one from someone else, you can only turn off Activation Lock by entering the correct Apple ID and password.” The correct Apple ID and password is the combination allowed by the rightful owner of the device. 

So, if you forgot the correct Apple ID or Apple ID password, use Having trouble signing in/iForgot  https://iforgot.apple.com/password/verify/appleid to identify either or both.

You can also use:

 If you forgot your Apple ID  https://support.apple.com/en-us/HT201354

If you forgot your Apple ID password https://support.apple.com/en-us/HT201487

 If you get far enough into the process, you may be able to Use recovery mode to restore your iPhone [iDevice]. If this step does not work, get help from Apple support.

We’re here to help https://getsupport.apple.com/?caller=kbase

Go here if you need additional support Apple telephone support https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362  

For worldwide Apple ID support go here https://support.apple.com/en-us/HT204169

Note that all recovery advice can only apply to the original purchaser of the phone. Apple will not assist the owner of a second-hand phone under any circumstances.

Incidentally there are sites which offer to unlock an Activation Locked phone for a fee - avoid: these are scams.