Yes, I think it would be inappropriate. As you pointed out, she is fully aware of her physical condition, and intelligent enough to make a choice.
If it were something not so noticeable, like perhaps she was depressed or something, you might consider talking to her about it. But in a world where we are bombarded daily with the message that being large is not OK, in movies, TV, billboards, magazines, newspapers, doctors' offices, restaurants and even accosted by total strangers (yes, it's happened to me and to others I know), you very well might be the only friend she has that hasn't
mentioned her weight.
Perhaps that's why you're still her friend.
At any rate, I would leave it alone unless she brings it up. If she does ask for your help, be understanding and supportive. If she doesn't... well, be the one person in her life who she can count on not
to chastise her for ordering something besides a salad and diet iced tea.
(I'm speaking from the heart here, as a large woman who is healthy and active, and still regularly receives glares, sneers, and arch comments. If I've said anything offensive, or made assumptions about your friend or your friendship, please forgive me. This is a topic very close to my heart.)