Thanks! Some of this might sound a little obvious, but here are some of my tips:
What is really important, is to make it clear to her that she is not supposed to walk around hungry or feel that she needs to starve herself. Eat food that is high on nutrition, but low on calorie. If you can, make shopping kind of a "family activity" that you do once or more a week, planning meals ahead and where the goal is to choose healthy food and steer away from the bad food. I do this with my mom and dad once a week now, it is really good for me to have control over the food we buy. My dad loves fat cheeses. In the start I had to tell him: please hide it, because I like fat cheeses too. Now my willpower is so strong that I can avoid it, but in the start I think it's best to just cut it out completely. Make dinners with interesting salads, she'll end up loving vegetables in the end. I know that personally I hated salad before, because I thought it was only the green stuff.
Make eating an own activity; not something you do in front of the TV, and don't eat right out of the fridge. Take time to prepare meals, and really cherish the food. Make her look forward to your planned meals instead - they actually taste better when you can't have unlimited amounts, at any time you want.
Losing weight is something she has to want for herself. She needs to understand that results won't come for free, but results DO come if she puts in the effort. Also comfort her with the fact that after eating a certain way for a certain amount of time, her body will get used to it. It won't feel like it's a struggle every day. Some days it WILL slip, and she'll end up feeling bad for it - but tell her it's okay to let it slip sometimes. She'll get back on track again, no harm done.
Exercise speeds up weight loss, so if she has a favorite sport, encourage her to play it. If she hates working out, slow/moderate walks help too. I walked a lot with my mother in the start, when I was completely out of shape. It was an hour every day where I spent quality time with my mom; just the two of us, walking and talking. I didn't see it as exercise. The support from her was so important at that stage. She said: "I know you have tried many times, but this time I really believe in you. This time, you will make it."
Oh, and tell her that the reason why she can't see progress is because she sees herself everyday. Being her mother, you also see her often, so taking lots of photos can help. That is a mistake I did, I didn't take enough pictures. A fun thing to do is to take one picture straight on, one from the side, and one from behind. Let her stand in the same place in every picture, wearing tight fitting clothes. Let her keep a little photo journal if she wants to, make collages and be creative with it. And remember to encourage and compliment her when she is doing good, that is something I wished I got more of in that period.