The options I considered:
1. Selecting 50 emails at a time, and doing 'Forward as attachment', to convert each email into an .EML file and attach it to an email.
2. Using Microsoft Outlook on his computer to connect to Gmail via IMAP. Then selecting multiple emails in Outlook, and choosing 'Print...' to 'print' 50 at a time to PDF files.
3. (best option so far) Use Google Takeout to get an mbox file, and then use https://github.com/frehder/mbox2eml and https://github.com/andrewferrier/email2pdf to convert the emails into a set of PDF files.
Although this isn't a very common need, I'm sure someone here has been asked the same thing by a friend or relative. How did you solve it?