I have an idea straight out of the HB's playbook. Although it likely has a low probability of success, here is a more long-term solution:
Make her your emotional tampon.
You haven't told us what your relation to this chick is, but regardless, here's what you do. Treat her like your sister. Confide in her all the women problems/dramas you're having with women who are hot and (most importantly) in shape. Since she's your great friend, point out when you see a hot piece of ass and tell her things you'd say to your buds. "Man I'd tap that!" You get the idea.
The whole idea here is to get her into a oneitis situation with you, but you've totally LJBFed her, and hopefully that crushed feeling she gets will be the emotional jolt she needs to make her realize that in order to get quality guys interested in her, she needs to get in shape. If she's not interested in you, perhaps she'll still make this connection. My idea might take some serious effort on your part, definitely more of your time than it's worth, but the only way she's going to lose weight and keep it off is if she finally realizes how being hot impacts her well-being.