You could consider telling your client to take the stairs everyday if that’s an option. I think a good exercise could be just getting up periodically and doing a few body squats, arm circles, a few toe touches. No more than 30 seconds or so if the work schedule is absolutely packed. Getting up every now and then, even for just a few seconds can do wonders for your psychological well-being, in my opinion. You might also tell your client to do some shoulder rolls or some light neck stretches periodically throughout the day (maybe make a figure 8 with the chin to loosen up that neck). Some arm pumps might be useful to get the blood flowing. I really like Karin’s suggestion about the pedals. As far as sitting all day, the best investment your client might could make is something that will offer plenty of lumbar support and cushioning for the seat. Posture can improve with habit of maintaining good posture, so I would be sure to emphasize that. Better posture leads to better breathing, which leads to a better sense of well-being.
Great question! Now that I think about it, MANY people have this type of job. You’ve got me thinking about this issue!