You know that old saying about the road to hell being paved with good intentions?
Has she actually asked you for help with her diet? Is she just a housemate or is she a close friend with whom you have the kind of intimate relationship that makes it acceptable for you to comment on her diet?
Unless I had asked you specifically for help I would find it offensive to find “helpful” notes and such popping up around the kitchen, especially in light of the fact that I had just lost 120 pounds.
Also, assuming you look anything like your avatar, and that she is not a mental defective, I would also find it somewhat hard to believe that you suddenly need a reminder on the fridge to eat enough protein during the course of the day and would assume it was aimed at me. It would stress me out, fat girls are extremely self-conscious.
And if they are relatively newly former fat girls it takes a while for the head to catch up with the body, i.e equally self conscious. All of which you or may not give a shit about, but you do have to live with her, and like you said it will cause tension, girl-gaggle bullshit, all those tears and hugging etc., (male roomates are really awesome, by the way!), Okay, I digress…
I am, however, a curious sort, and if continued fat-loss/health/fitness were my goals, and I lived with someone who looked like you, it would certainly occur to me to keep an eye on your habits. Therefore, in my opinion, leading by example would be a really good bet.
If your Grow! (really need a lady version, maybe call it Shrink! Far more appealing?) were sitting out on the counter, you can bet I would get around to checking it out in short order, followed my questions, followed by request to taste, and so on…