They would have to give you their login e-mail id and password.
You could then login as them and change them as suits you.
[If they use duplicates of other accounts they have, e.g. same e-mail and password as their own personal ones, they will want to change from them on Plarium first.]
IIUC, facebook / apple / google only allow one player character per platform, so if you already have one of these, this option isn't open to you.
It seems this is how joint mastership works too. i.e. Someone able to maintain the administration of an order while one or more are away or sleeping - joint shared ids and passwords. Requires a lot of trust between them. Which I've seen go sour, to great unhappiness, more than once.
The only 3 things a master can do that a Counsellor can not:
- change the order code [So change the order info to point elsewhere, instead. e.g. "Ask a counsellor for the link."
- demote a counsellor [If they're inactive, it don't matter.]
- distribute OvO winnings. [Not germane. Amounts not large enough to matter - if they are, you're in the wrong order / seek another.]