If your dog likes green beans enough for you to use them as treats, that's one thing you can do. They are certainly not fattening.
Try other vegetables as well, as long as they are good for dogs. (you can look this up online)
My dogs love carrots and one of them also likes celery and kale. I had a foster dog once who would work hard for a piece of lettuce!
In addition, cut back on the amount of food you are giving her. Not drastically, just a little tiny bit, since she is a small dog She probably won't notice, and you don't want her to lose weight too fast in any case.
And most of all, take her for longer walks. Exercise is the best way to lose extra weight. Much better than diet, for dogs and people both.
finally, you can check the fat percentage of the food you are giving her, and perhaps choose one that is lower in fat. But avoid those "diet" foods because often they are not nutritionally sound.