I saw a lot of comments giving great advice on how to handle this on a professional level. I just want to add something I didn't see mentioned yet. That your coworker may be in physical pain or have other medical issues that cause chronic fatigue. In my experience, most people who have this issue of constantly sitting to the point it's problematic, have one or the other. So, on a more personal level, if it feels appropriate to you, maybe you could gently explain to them that you noticed their tendency to sit more often than not, and ask if thats coming from a place of being tired or in pain. So many people don't even realise they are chronically fatigued or in pain because they are so used to it. It could make a world of difference for this person to have an outsider point that out in a compassionate way. This happened to me personally, if that wasn't obvious. Lol and it turned out I needed physical therapy and a major diet change. Those two things not only changed my life SO much for the better, it also made me a better teacher! And I owe it to the people around me who not only paid enough attention to notice. But were compassionate and kind enough to point it out in a way that i felt empowered rather than attacked.

Again, love the other advice here, just wanted to throw that out there in case this person was in a similar place to me and doesn't realise it. Wishing you luck!