There is a tactful way, just say that you realize she's going through tough times or maybe thinks she's fine the way she is, but that if she started walking daily and eased into running not only would it make her feel better, it would make her healthier, and give her more confidence (make her feel sexier!) tell her at first it'll be hard, but you'll help her ease into, and after a month of making a habit of it, she'll be good to go.
all my amigas know i run a lot, and i try to get a lot of them to start running too, especially if i can tell they need it. one of my girl friends started running, lost some weight, and looks so attractive now and is still running! :-) another of my friends is just starting up too!