As a chubby girl myself...
As a chubby girl myself, I can tell you that suggesting that she lose weight would more than likely hurt her feelings.
GreenGiant had the right idea about inviting her to the gym. She may make a little, "are you saying I'm fat?" joke, but more than likely she'll take it as a way of you wanting to spend time with her... introducing her to things you do with your time. (Try that... "I spend a lot of time at the gym. I'd really like to see you before Friday... I'll be at the gym on Tuesday, you should come with me.")
In getting to know her, you'll pick up on her physical mentality - whether she's healthy and just needs to step it up or if she just doesn't care, period. The "I don't give a shit" attitude is obviously not attractive, but the "douchebag called me fat" approach is not either.
Even if you don't mean it like that, even if you're not shallow... this girl may be in your life for quite some time and you'd hate to hurt her feelings by a comment about her looks. (Trust me, she'll never forget it.)
It took me a while to even get naked in front of Jrod... even after him expressing how pretty he thought I was. And talk about my weight/size was unmanageable.
After seeing/helping him workout, our conversations started to change. Eventually, I started working out with him. Now, we talk about all kinds of things... nutrition/fitness goals, problem areas, etc. His desire to be healthy is attractive to me and it makes me want to be my best self, too.
I definitely have my "do you still think I'm pretty?" moments... just like many other girls, but he's awesome at making me feel like my curves are sexy.
Needless to say, I rarely have clothes on around the house now.