What should a 12 year old that is 5'2 girl weigh?

Anywhere: Ideal weight depends on multiple things but generally they are somewhere between 105-120 lbs. You can calculate ideal weight at this site: http://www.Medindia.Net/patients/calculators/height_weight_forchildren.Asp.

75th %ile: 5'2" is just above the 75th %ile for height. That corresponds to about 106 lbs of weight at the same 75th %ile. Growth patterns may vary. Also, some people have larger or smaller frames. This answer is merely an average.

IBW: Male: 5 feet=106 lbs. Thereafter, for every inch add 6 lbs. Female: 5 feet=100 lbs. Thereafter, for every inch add 5 lbs. In your case, it should be 110 lbs. Hope it helps!