is 1200 too low?

[strawberryyDaiquiri]

My sister is about to start on 1200 a day, but IMO it's a bit too low. My logic is this (please don't hesitate to tell me I'm talking utter cr*p! I probably am..but these were my initial thoughts)

She wants to lose about 2 stone. If she starts off at say 1600 calories and loses weight, once she hits a plateau, she can drop down to 1400, and she will then continue to lose weight. If she hits another plateau, she can drop down to 1200 (never lower) and add in a little more exercise.

Is this logic correct at all? What happens if you hit a plateau on just 1200? It would be dangerous to go any lower so would you just add in more exercise, eliminate food groups? Or maybe up calories for a couple of weeks and then drop back down again?

Maybe it would help her if she joined something like My Fitness Pal? I have just returned to using it again after a break of almost a year and it is great in that it works out a safe and appropriate calorie target based on your height and weight. It also encourages you to eat back exercise calories so that you are eating enough to sustain yourself while losing weight. Admittedly I don't always do this.
I finally got a smart phone a couple of months ago and so now have the app which I hadn't been able to use before. You can link exercise apps like Map My Walk or Map My Run so they automatically add your exercise calories to your daily food/exercise diary. It also has a barcode scanner so you can scan products to ensure you can log the correct nutritional value. Keeping a food diary can be a really good way of ensuring that you don't accidentally over-eat or are slipping in food that 'doesn't count' (e.g. children's left-overs, cakes at work) and also make sure you are definately eating enough.