How thin is too thin?

Ok, my best friend is real skinny. And she is...*laughs* ..really vain as well and she likes to tell me she has nice legs. I agree...she has thin legs...but she is really bony and she eats like a HORSE! I feel myself getting bloated just watching her scarf white bread like its no big deal..:D. But how do I tell her that she needs to stop eating these things because her metabolism will not be this way all her life, and it is starting to slow down even more now. And how do I tell her nicely that she is being too vain about her body and she needs to stop flaunting? I know it might sound like I am jealous...but thats because I rather am! I am 97 pounds and curvy...but I am still jealous of her boy shape. Please help!