I have to give my girlfriends the benefit of the doubt.
I too am the skinny with my two best friends. (And check out my weight...I started at a BMI of 40) My friends complain about their weight and I do see them eating things they shouldn't. I DO want for them to find the same benefits I have to losing weight. BUT I just tell myself it's harder for them, there's something wrong with their metabolisms, and I tell myself that because it's none of my business what they do with their bodies and I know that the only reason I want them to join me is because I want it for me. I selfishly want them to do this too. It's a bit braggy, you know?
Now, if/when they bring it up, I'm all over it. And they do bring it up and we talk about the trials and difficulties and we brain storm together. But ultimately it's their decisions and ultimately I get to just love them for who they are.
Anything more than that and I become the judgmental one who has a problem with their weight. I suddenly think I'm better than they are because I lost the weight.
That's the harsh reality of how I think your friend would take any push from you.