Actually, I'd heard radio reports indicating that sitting for 10+ hours on a trans-Atlantic flight can promote DVT and that getting up and moving around somewhat is highly recommended. The "exercise" need not be particularly vigorous; what's important is basically that the person move his/her legs every few hours.
Seems like very reasonable advice to me; if carried out sensibly it won't cause any problem or distress for other passengers and crew.
Planes are incredibly tight on space, particularly outside of first class. 1) Is she going to repeatedly climb over the people near her to get up and exercise? 2) The aisles are usually occupied by the flight attendants and the food cart. 3) Do you want someone in the aisle beside your seat exercising? I don't. 4) Ever heard of air rage? Given passenger anxiety since 9/11, I don't think I'd want to be doing exerises, irritating other passengers, and making them think I had a screw loose.