Two words: Slimming World.
My adopted/surrogate mum type has lost loads of weight with Slimming World, she's literally half the woman she used to be. By comparison, my actual mum has tried weight watchers and is now the biggest she's ever been, because she didn't stick to the points thing.
Plus Slimming World is fairly easy to understand, and is a way of learning to eat sensibly and healthily, it's not just about losing the weight.
Excercise always helps too. It is also advisable to see the GP, just to check things over and to decide on a suitable & healthy target weight.