What should they have done?

-Stuck to Her Good Diet.  This means: three or four meals a day at set times, with meat, fish or eggs and plenty of vegetables at lunch and dinner, a small amount of starch, fruit and a dairy product.  Whether you’ve just set up home with your partner, started a high-pressure job or moved to New York, stick to your good eating habits so as not to put your body under any further stress.  
-Organised Herself.  To stick to an eating routine, you need to manage your new lifestyle.  If you’ve moved abroad, pick foods you know to get yourself back into your good habits.  If you’ve started a new relationship or moved in with someone, try not to change your eating too much, just restrict portion sizes, especially fats and starches.  If you’re snowed under at work, make sure you build rest breaks into your day, even if they’re short, and make your lunch the night before to avoid any stress.
-Take Action in Time.  Once you start putting on weight, it’s hard to break the cycle.  Keep an eye on yourself by stepping on the scales regularly if you’re going through a life change – at least once a week, on an empty stomach, with no clothes on.  That way, as soon as you notice a change you’ll be able to take action.  It’s easy to lose a few pounds than a few stones!