iCloud activation lock

Has he tried : If you forgot your Apple ID - Apple Support ?

Or, and if he was the original purchaser of the iPad (i.e. it’s not second-hand), he could try going to an Apple Store (he should make a Genius Bar appointment if he does) or try contacting iTunes Support online (https://www.apple.com/emea/support/itunes/contact.html) and see if they can remove the iPad from the account - they are likely to want to see proof of ownership before they might help e.g. original receipt with the serial number on it

He should go to https://iforgot.apple.com to retrieve the ID and reset the password. See also If you forgot your Apple ID - Apple Support

Failing that, and assuming he bought it new in the first place, he 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.