Gnus has been great for me, but after about 40k+ (I stopped looking at the counts actually) messages in my Maildir, performance suffers for me (that is, if I start gnus from scratch it takes more than an hour and a half to open, with my entire Maildir in the page cache, and a huge amount of extremely fast memory). Notmuch doesn't seem to help at all, or if it would, it's hard to tell if it's configured correctly.
40k email? That sounds too much if it is for a single person. If you get 10 email per days for 10 years, you'd not get to 40k (35.6k). My suggestion will be to clean your mailbox instead of finding a new client.
I recall times (mid 2000's) when I was active in a bunch of open-source communities and mailing lists were the main way to discuss. Getting around 50 e-mail per day was normal.
I kept a lot of those discussions in my e-mail client for later reference, because the mailing list website had sub-par search function compared to Mozilla e-mail client I used at the time.
It's possible he's using it as an e-mail archive for mailing lists or something like that.