Ipad Air 2 security locked and no access to apple ID password

We lent my elderly mother my iPad which sent back to factory settings and set up for her as new with her own Apple ID and password. She has Parkinson's Disease. Somehow she managed to change the password, she does not remember doing this and now the ipad is security locked. It was originally purchased from John Lewis in 2016. Apple will not accept the original purchase invoice as proof of purchase because it doesn't have the iPad's serial number on it. John Lewis tell me they do not keep records of serial numbers or put them on invoices and so cannot help me at all. This is what they sent me.

Response By Email (24/05/2022 16:52)

Our reference: 220523-000053 

Thank you for recently contacting our Head of Customer Service regarding your iPad. Your case has been transferred to the highest point of escalation within John Lewis and will be handled by myself as a Customer Relations Manager. 

Unfortunately, we cannot provide a receipt with the serial number. Only Apple follow this practice. However the proof of ownership can be shown through your Apple ID. As this shows any apple devices that is registered to your account.

Furthermore, you can unlock the iPad yourself. There are plenty of guides and videos that shows how to do this including on the Apple website. I hope the above proves helpful.

Can someone tell me where the guides and videos on the Apple website are that will allow me to unlock my iPad? I don't believe that the information John Lewis has given me is correct. Apple will not remove the lock without proof of purchase plus serial number and I cant provide that if John Lewis wont give it to me.

Thank you