What should they have done?

-Chosen Their Diets More Carefully.  Generally speaking, very restrictive diets (less than 1000 calories/day) are a really bad idea, as well as unbalanced diets which cut out whole foods groups (protein diets, vegetable-based diets, dairy-based diets) and miracle pills or potions.  Don’t listen to anyone who promises a new you in return for a fat cheque either…
-Had a Realistic Target. If you need to lose 2kg, than aim for two, not five!  Your body has its own ideal weight, at which it can function properly and look its best.  Losing too much weight will only lead to fatigue and piling on the pounds afterwards.  Take your time as well, diet which ‘work’ too quickly won’t give good long-term results.
-Stayed Vigilant When They Reached Their Target Weight.  This is the most important time of your diet, when you have to try to start eating ‘normally’ again, without putting back on the weight you have lost and undoing all your hard work.  Make sure that you don’t get complacent, which means no eating too many cakes or rich sauces, keep an eye on portion sizes and don’t get carried away with fats and sugars now you are slim again!