We had a similar colleague that had issues, but their car seat is folded almost flat onto the back seat. So it made it worse. Best is to send them to a doctor dealing in Ergonomics.
They tried a few chairs and then eventually we liked the Geocloud Step Ahead chair. We have since replaced all our company chairs for those. It's about R3500-4000 per chair.
They are a local company. Great to deal with. Do desking too.
Executive task chair shown with upholstered headrest, netting backrest, adjustable lumbar support, height adjustable arms, upholstered sliding seat
www.geocloud.co.za
For monitors however we didnt like the monitor brackets. Didnt get from them but from another supplier before. Tossed all the brackets and went for the Dell P series Monitors ie P2417H and P2717h. Can swivel, tilt 90 degrees and raise up and down. Since then we have also switched to only those. No need for the brackets thats difficult to move around.
You can also look at getting an ergonomic footrest. Some like it, especially if they are short.
Lastly Orico sells nice big mouse pads. Works well. For keyboards we did like Microsoft Ergonomic but most prefer the plain cheap HP (or Dell) Slimline keyboards and then we use Logitech Cordless Mice.
The biggish mousepad offers wrist protection.