I echo Peter's question, and would like to add a variation of it -
My brother has given me full permission to use his DNA results, and do anything I want with it. I inform him of everything I do, and show him any results, but he has not shown any interest any managing or doing anything himself with it. I finally got him to click on an invitation, and he put in a password and sent it to me. He knows he can modify anything he wants, but wants me to manage it, including the DNA info. May I log in as him and add his DNA? He's an American citizen, not ever leaving the USA (not interested in ever getting a passport), has given me full permission, and wants me to do what ever needs to be done, so there is no question of any miss-compliance with the GDPR.
Before someone says "Shut up and just do it" - I don't do things that are considered wrong (I'm brain damaged that way). But it's sometimes not wrong to log in for someone else, for example, for computer troubleshooting or training. You are logging in as them to perform various actions or demonstrations "on their behalf", with their full knowledge and permission. So what I am asking above is the right to act on my brother's behalf, because he has given me full permission to do so.