Father with Dementia

Hi. I'm trying to change my father's email address and Password to a new one we can use to manage his account going forward as he has Dementia. When I arrive at the 'link accounts' screen to Register for online services so we can manage his Virgin Media, it finally shows his old (over 10 years old) Blueyonder email address which A. We can't access and B. We don't know his password for. So we can't then change ANYTHING on his account. ALL we want to do is -

Set up a new password and email address on his Virgin account.

Delete his old email address completely.

Register his account online so we can amend his package (what's insane is we want to probably ADD to it, and Virgin are making it almost impossible).

Can someone PLEASE help us, as we have tried online and the customer service is appalling, they said 'we've changed the email address' twice now and it's STILL coming up with his old Blueyonder email he must have used years ago to sign up. We are pretty desperate trying to sort things for him and dealing with Virgin is beyond difficult.

Thanks