What's Eating Gilbert Grape?

So I love eating.
I love tasty nachos and good New York pizza.

A friend of mine, she's a little bit bigger than I am, and we love going out to eat together. We love ALL the same foods.
Well I guess she's more than a little bit bigger... and my husband thinks it's unhealthy for her to eat those foods. I guess it's because when I eat those foods it's once in a while, as a treat, but she eats bad food all the time. Anyway she's really his friend first, they've known each other forever. So I guess he's worried that she 's put on quite a bit of weight and that it's really bad for her body.

My mother-in-law even tells me about how unhealthy she's become. My mother-in-law has also known this girl for over 20 years. She asked me to speak with her. She wants me to tell my friend what to eat and what not to eat. She says she can't do it as a parental figure, but because I'm her friend that I should.
And if I don't tell her then who will?
But what am I supposed to do?
How does one say that?
Any advice?