Let's go on MAury and get you a makeover!
Well, I saw a lot of girls wearing stuff that was so inappropriate for them and a lot of it boils down to wanting to fit in and not knowing that just cuz they make it in your size does not mean you buy it.
Why don't you talk to her as one BBW to another privately? Tell her how sometimes you struggle to find clothes that compliment your shape. Give her some suggestions of appropriate clothes. Tell her you are only talking to her because you care. Offer to work out with her as well to help shed some of the weight. Look through magazines and find clothes that will flatter her -- both body weight and age.