Logging in requires not only the username and password but also something like a code sent to the mobile number of their phone.
If they have lost/forgotten their number and/or lost their device there are two options. If you are a CSP provider and manage their tenant you should be able to go in through Partner Centre into the customers tenant and do the reset. If you are not a CSP and don’t have Partner Centre they will need to reach out to Microsoft Support in order to help resolve the issue (in this case get their verification number changed to a new mobile number or get two factor disabled on their account so they can get back in)
@Christopher HoardIt has now been resolved, the customer has received a response from Microsoft how to make changes to **** with his IT supplier to confirm that he owns the account. I got the feeling that he is still happy after contact with Microsoft.