previous owner does not remember their apple id for activation lock

Hello,

The iPad is locked with Activation Lock. There is no way around unlocking Activation Lock without inserting the device’s associated Apple ID. If this is not an option you will have to find the original receipt for the device that clearly shows the serial number. Once you have this document you can have Apple unlock your iPad.

Activation Lock

Have your brother read these documents and try to recover his information. That is really the best solution:

If you forgot your Apple ID (your Apple ID is usually your email address) - https://support.apple.com/HT201354

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

https://iforgot.apple.com/password/verify/appleid

What to do after [or before] you change your Apple ID email address or password - https://support.apple.com/HT204071

If you still need help:

Contact Apple for help with Apple ID account security "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues."

https://support.apple.com/HT204169

or

Contact Apple for support and service - "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232

Otherwise your brother needs to do this:

Apple may possibly remove a lock if a person can provide sufficient evidence of valid ownership. The device is normally taken to an Apple Store with identification and the original sales receipt. I.e., the receipt when the item was sold to the first owner, and preferably by that person. There is apparently no set policy (I asked at an Apple Store) and a store may simply refuse, particularly if they are not satisfied you have proven ownership by provenance.