Losing weight shouldn't cost you money, it should help you gain some... If you're overweight it means you eat too much for the needs of your body. By reducing those fatty foods (so fat) which your body has enough of at the moment and replacing them with proteins (fish, yoghourt, skimmed milk...) and as many veg and fruit as you want (without anything added) you're well on your way to a healthy diet. Wholemeal bread is good too. Add to this a walk or swim every day and within 3 months you'll see a great change.
You can still have a cake or chips but it's a treat now and again not a daily thing or you're back on track to being overweight.
By the way £150 per month is a scam but if it convinces her she'll lose weight and quite honestly at that price I'd make sure I did, why not????!!!!