I was looking "naïvely" for the button to request all of his personal data. I didn't find one and there's probably one somewhere I'm guessing.
I resigned myself to set up Outlook on his computer and make a manual backup.
I also use it to save Spam/Junk folders, which then comes in very handy to train my local spam classifier for my self-hosted mail servers with lots of data. (Over 3TB of spam saved so far and about 20GB of ham.)
Gmail’s spam filter has had a higher false positive rate than usual for me lately, so I have a little report emailed to me once a week of likely ham in my gmail spam box, which has found at least 3 messages per week that I missed.
The spreadsheet keeps log of each msg in a thread.
Labels marks the downloaded emails.
Apps script run on a trigger & does the heavy lifting of actually downloading the .eml.