Sometimes it's because they are missing something in their diet. What food is she eating? (Including treats, table scraps, etc)
There are some pills you can buy at just about any pet supply store. Poo eating is termed coprophagia, so this is what some of the boxes may say. I have no idea how well or terrible these work, so can't help you there. If it's not a dietary problem, then it's just behavior. You can work on this by developing a solid "leave it." This means you'll have to watch her like a hawk when she's out to potty. Clean up more often than usual so she doesn't have as much opportunity to eat it. Good luck, hope this helps!