I concur--she needs to motivate herself. You can help by not focusing on her size (that way she won't focus on it as much herself) and concentrate on how much better she feels.
I'm a BFB (Big Fat Broad). I think free weights are great for heavy women. A lot of us weren't athletic as kids. Some got heavy after pregnancy. The nice thing about free weights (and I'm not talking the kazillion reps with a pink dumbbell here) is that you see your improvement every time you can increase the weight, sets, intensity, etc. You see yourself getting stronger. You know that you're getting stronger because your muscles are working. If you're eating right, then you're putting on some LBM, which means your metabolism is increasing.
Also, help her set very teeny, tiny, baby step goals on the way to her first large one. For example, she shouldn't try to lose more than 1 or 2 lbs. of weight a week (I know, I know, there's a difference between fat and weight loss, but stick with me here). Therefore, a good mini-goal is that first 5 pounds. A good medium goal is 10 lbs. A good long-term goal could be 15 or 20 lbs. (I suggest you go with body fat % changes).
Measure her. Sometimes the "weight" doesn't come off, but you will lose inches.
Set goals for improving her lifts, too.
That's all I can say right now--on the way to the gym! Good luck!
Oh, yeah. Get her to read this board if you can.