No, it won't harm her. Amprolium is just a thiamine blocker [cocci thrive on thiamine]. I would also give it to the rest of your flock. The treatment is for five days and do not give any supplemental vitamins while on amprolium. After the five days, give the flock either Sav-A Chick electrolytes or Nutri-Drench for poultry and I like to give probiotics while I'm at it.
If you find the 20% powder, its 1/2 teaspoon per gallon. I'm not sure about the quantities for the 9.6% liquid form.
If you find the 20% powder, its 1/2 teaspoon per gallon. I'm not sure about the quantities for the 9.6% liquid form.