What's your budget and can you maybe probe if there's one he's already looking for? There's huge variety in what people like for peripherals especially if they use them for hours at a time, so even a great quality mouse might not be the right fit for your partner. Razor is known for not having great quality / products having them break shortly after warranty, so I personally try to avoid them.

The easy to clean part sadly also contradicts having many buttons but imho it's best to clean your mouse as soon as dirt starts to build up, so maybe add a little toolbox for cleaning with a bottle of isopropyl alcohol, a toothbrush, some q-tips and a lint-free cloth.

I've been using a Logitech G502 for years now, it's not fancy but very widespread and even though mine still works perfectly, there's tons of replacement parts online and disassembly to replace the most stressed parts (left/right mouse buttons, scroll wheel and glide pats) is really easy to do. At some point switches will wear out, since it's constantly being used so repairability is definitely helpful. Also the software isn't too terrible in comparison, which is another point, why I wouldn't recommend Razer or Corsair peripherals.