I've got Ortliebs, but the bike packer plus. I got those because I liked the top closure better, and really like the side pockets for quick access to a few items.

The attachment system on the Ortliebs is very robust. I've had mine for 5 years, commuting by bike 50-100 days per year, and did a couple tours, including a 10 day from Eugene to San Francisco. No breakages, and nothing looks like it would break easily. Still water proof.

Not sure that helps, just my n=1.

I really like my Arkel bag. The attachment to the rack is quick, simple and very secure. I also like that they're made in Canada (though not sure where the Ortliebs are made these days).

I've had my ortlieb bags for 7-8 years of commuting. Still fine.
I get the feeling both are are very good and finding which is best is a matter of taste

One last piece to consider: just got the REI sale catalog and come 5/19 all Ortlieb panniers are 25% off.

Ortlieb. Always Ortlieb.

Even if Arkel didn't make most of their stuff in Canada from American sourced materials, I would recommend them. I have both Arkel and Ortleib, and I prefer my Arkel bags (and I love my trunk rack).
No matter what you choose, make sure he tests everything before the trip. It can be as simple as riding 50 miles then setting up camp in the yard, but he won't realize the flaws in his system until he tests it.