I assume he is going to change Internet Service Provider. When he does so the AOL eMail account can be downgraded to a PAYG - Pay as you go account - the term is a throwback to dial up days but the account will stay active as long as it is used. Therefore all his eMails will still be there.
To download them from the AOL server. Using Outlook, if that is your prefered eMail client, and download in batches. Then Save the messages to a New Folder so they are moved out of the Inbox to allow the next batch.
Make sure the messages are saved as.eml files. If you save the pst file you may not be able to access it in the future, different eMail client, different PC.
I am thinking about how to create the batches, using filters, maybe.
Can the messages be saved directly from the AOL screen to the local PC?
Does he really need ALL those eMails, for 10 years, even Inland Revenue only ask for 7 years records and are any junk that could be filtered out before downloading?