Hi! Any diet that isn't balanced (ie: is based on one food category only) may make you lose weight fast, but as soon as you stop doing it, you will put it - and more- back on. Also, you will lack a lot of nutrients (in this case, mainly proteins I assume). Is this "fruit diet" based on fruits only?
Your body is designed to store fat, it's a survival instinct. If you starve yourself, the body will go into emergency mode and store as much fat from whatever you eat as it can. Which means that once you start to eat normally again, your body will store up more fat from your food than it would have if you hadn't starved, hence putting the weight back on after a "fad" diet, and even more than you originally lost. I hope this makes sense.....:-)
The only way to lose weight safely, and durably, is to eat sensibly: plenty of fruit and veg, avoid saturated fats but do not cut out fats completely, drink plenty of water, and exercise more. Cutting out alcohol is a great step!
I wish you all the best!