How much weight should be lost?

So my cousin is 5 feet 6.5 inches or like 168 cm and she weighs 148 pounds or 67 kilos. First off do you think it's fat? I mean her tummy is thin and about flat but her thighs are well rounded and she has hips. Anyway, she qants to lose weight. She has a medium type frame and wants advice on how much to lose and how to lose it. Any help is appreciated. Thank you!