I have a friend who says their ideal weight is 70 lbs and they are 5'2". It makes me very concerned, is 70lbs even a possible weight at that height? I'm pretty sure my eight year old sister is just under 70 lbs and she's around 4' and skinny. I can't imagine 70lbs is possible on a 5'2" frame and if it is I can't imagine it's healthy... Do you think maybe she meant 70 kgs?

It is probably possible if the drop is gradual and not something crazy drastic. It seems ideal for someone who has anorexia and/or a very severe body image problem. Or, who wants to die....

The rule of thumb is 100lbs for 5 feet then 5lbs more for every inch. Usually this rule will put you at a healthy bmi point. So at 5'2 that would be 110lbs.

Seventy pounds is considered very underweight for 5'2 with a bmi of 12.8 on the standard scale and a little over 13 bmi on the new scale. Even if we assume she is small framed or consider ethnicity differences, she'd still be severely underweight. To put this in perspective, Korean Pop stars like Ne21 or Girls Generation tend to have a bmi of 15-17. I've also had a friend who was 4'11 and claimed to be ~90lbs, she was already small framed and to the point where she was sized out of most American adult brands.

Now if we assume she meant 70kgs, that would be 154lbs. That would put her as solidly overweight by bmi, closer to obese than normal weight and almost obese on the new scale. There are diseases, situations, and frame sizing that may cause someone to be slightly heavier but that is why bmi is a range. Even considering she did have health conditions or she is very large framed, she would still be at the top of the bmi normal weight or on the lower end of the overweight range. Either way in both situations her weight would not be ideal, but 70lbs is far more concerning than 70kg at that height since she'd probably be bordering death and have a multitude of health scares, though both need work.

That sounds like anorexia. It's a tough illness to cope with. You can read up on advice for how to best support her. There may not be much you can do, but at least you'll know what to avoid. How old are you, are you still living with your parents?

Was she serious? Or was she is in a depressed/sarcastic mood? It's hard to think of someone meaning that seriously.
I guess if you want to be emaciated?

Even 100 pounds at 5'2" is considered underweight on the BMI scale. 70 would almost certainly be grounds for hospitalization. If she was serious and meant what she said then that's a real problem.