The best way to lose weight is to exercise, do aerobic exercise or something, and then eat something light, don't deliberately diet. This is not good for your health. Eat more fresh fruits and vegetables, and then you can do a good job of supervision and help your girlfriend put some pressure on her. Appropriate pressure is motivation, and you should also give her appropriate encouragement, such as giving her something she likes, making her stick to her firm beliefs and exercise well, so that I think it is not far from the slim figure your girlfriend wants.