my ipad seems to be logged in with an unsupported apple id

the issue is this: my elderly father can't log in to his ipad anymore. He bought it a year ago at an official reseller and let them install it for him. Now, he claims, the ipad starts asking for his password.

I looked, and asked what his password is for *******@icloud.com. He says he never logged into the ipad with that id but rather with his ******@hotmail.com apple id. So he also doesn't know a password for that icloud ID. When I used my iphone so reset the password for this id, I get the message that the ******@icloud.com apple-id is not supported.

It also claims that "someone" activated that ipad in the "Search my" function so until that has stopped we can't log in with another ID.

To be clear: he has officially bought this ipad, has the receipt, but the people there are unable to help him.