Sounds like a fad diet to me. If it does exist, it's probably natural lemonade they're referring to, no? Generally any diet that suggests you focus on eating or drinking one thing through the entirerity of it isn't good for you. Plus I can only imagine drinking fizzy drinks all day will rot your teeth, and if it is natural lemonade they're talking about, then it's bound to raise havoc with the acidity levels in your stomach - bad idea.
I'd recommend cutting back on the amount of carbs you eat (potatoes, pasta, rice, bread, biscuits etc - although don't cut out completely), and try not to eat any carbs after 6pm.
Also, a can of red bull/energy drink can boost the metabolism and works for me if I have one a day (although don't drink more than 1 a day, or every day if it gets your heart racing or anything like that).