Brysing is correct. There are no spells that will help you magically lose weight. The only way to successfully lose weight, and more importantly to keep it off once you reach your goal, is to exercise and to eat properly. That means portion control, eating a balanced diet that contains small amounts of lean meat, fats, and starches, and lots of non-starchy vegetables. Weight Watchers is a good program for accomplishing this in a healthy way. She won't lose weight fast under the program, but she is more apt to learn good eating habits that will help her lose consistently and to keep the weight off.
Now, even though there is no magical spell to make one lose weight, you might look at spells to help her keep her will power strong and make it easier for her to resist the temptation to eat things she should not.
One last note, sticking to a diet plan is hard enough but it becomes even harder if the people around you are not supporting you. If you're telling your friend that you think her participation in Weight Watchers is "stupid" you are actually making it more difficult for her. Of course you are welcome to your opinion, but for your friend's sake, keep that opinion to yourself when you are with her. Support her choices and help her stick to her plan, that is worth more than gold when someone is struggling with their weight.