I don't know if I'd say anything directly, but you could always suggest activities that involve some form of exercise, like, "Hey, Friend, I just joined an Aquafit class at the YMCA, want to come with me?"; or "Why don't we go for a bike ride on Saturday, you, me, and Son? We can make a day of it, I'll bring a (healthy) picnic." That way, it's not criticism, but you can maybe help steer your friend in a positive direction, by providing fun, non-threatening physical activities to do, and healthy food choices, if you go for the bike ride/picnic option. Anything involving water is usually a good bet too, because it's low-impact, so it won't hurt her feet.