My 12 year old daughter is concerned about her weight. she is 80 pounds in weight and 53 in height. any tips for gaining weight?

The basics: The measurements cited place her at 90% for height for her age but 25% for weight. This asymmetry also places her at a body mass index (BMI) at the 3% for age/gender or underweight. In all likelihood she will put on more body mass naturally as she transitions into puberty. Exercise can/will trigger appetite & build muscle. A focus on higher quality meats, vegetables & low fat foods can help.