My wife got me some paint holder stands, they work pretty good. You just have to know what types of bottles he has (Citadel has mostly shorter, wider bottles, and companies like Army Painter & Vallejo have thinner, taller bottles). You put them together and with some wood or Gorilla glue, you're in business!
Here are some links to the organizers that I have or are similar, of different sizes and prices:
https://www.amazon.com/Amazon-Basics-bottles-thinners-brushes/dp/B08JMT7KD8/
https://www.amazon.com/PLYDOLEX-Paint-Organizer-Bottles-Brushes/dp/B09HZTNDKW/
https://www.amazon.com/Broken-Token-Modular-Paint-Straight/dp/B07MZ8Y4MK/
https://www.amazon.com/Vallejo-Front-Module-Paint-Stand/dp/B01720DWKU/
https://www.amazon.com/Vallejo-Corner-Module-Paint-Stand/dp/B013WYJ4CA/
Here's a wet pallet that I can attest to. I love it!
https://www.amazon.com/Masterson-Sta-Wet-Airtight-Supplies-Number/dp/B09PMQXW37/
Paint brushes are always wanted ... you can get a bunch of cheapies, or you can give a major gift of some of the best there is. Better tools make better art, hands down:
Very expensive:
https://www.amazon.com/Winsor-Newton-Kolinsky-Sable-Brush/dp/B00A6W345U/
Midrange Price:
https://www.amazon.com/Vinci-Watercolor-5359-Multiple-Brushes/dp/B0069FC9FO/
My favorite inexpensive brush sets atm are:
https://www.amazon.com/dp/B07GWYWV5D/
If you're investing in expensive brushes for your partner, then you should also get some brush cleaner to keep them nice, and they'll last a very long time:
https://www.amazon.com/Masters-Brush-Cleaner-Preserver-Soap/dp/B00F38YVQS/
The biggest present would be an Airbrush, of course. I find an airbrush to be indispensable, and I absolutely love mine. I always prefer to prime my models with an airbrush, it's a far superior and easier method. I also do basing and color blending with it. It's amazing.
I inherited a kit, the cheap kind from Amazon, that had a crappy airbrush, air compressor, and some other stuff. I learned on that, but the airbrushes were not good, and it made learning even harder. I went ahead and recently bought a brand new airbrush pen, the "Harder & Steenbeck Infinity CR 2" which came with two different needle sizes and adaptors (not all airbrushes can use different needle sizes, so this saves money, imo). I almost immediately got to the fun part of air brushing with it and it's very enjoyable to use. It's on the high end, though ... and that's expensive ($320). Not sure what your budget is.
https://spraygunner.com/black-friday/harder-steenbeck-infinity-cr-2-in-1-0-15-0-4mm-v2-0/
There are a few ways you can go with this, though.
You can go with a super cheap kit (this is essentially what I started with):
https://www.amazon.com/Master-Airbrush-Professional-Airbrushing-System/dp/B009YNG21U/
A more high end kit:
https://www.amazon.com/Steenbeck-Infinity-Miniatures-Compressor-Spraygunner/dp/B0BCH4D8CS/
There are also some relatively new air compressors that are handheld and rechargeable, but can get the job done and are a cheap alternative to an introduction to airbrushing:
https://www.amazon.com/Airbrush-KIBEE-Rechargeable-Handheld-Compressor/dp/B081HBK5K3/