i'm a big girl and i know that people talk about me like you have your friend and i'm sorry but putting photo's of slim women round the house will not work. if you feel like you say you do when your apart then you need to grow up, stop worrieing what people will think and go for it, what have you got to lose, if you don't tell her how you feel and give it ago you will look back and regret it.
BUT you need to get over the looks thing, if she was unhappy with herself then she would do something about it, she doesn't need her friends talking about her image, there will be enough people already doing that.
P.S i jump on my boyfriend all the time and hes still here and we've been together 3 years so i wouldn't worry about that.
good luck x