They won't backfire. Simply the opposite of what is said will happen, so she just has to say things in the right way.
Her ability was said to be unpredictable in how it reverse a situation, and is part of the danger it poses. If you are starving and she says "you're hungry", she could cause something simple and make you find money for food, or she could make someone giving you food for free, but she could also cause a chain of events where you end in jail and get food from there. Or you end dead so "hungry" doesn't apply anymore.
Her ability was said to be unpredictable in how it reverse a situation, and is part of the danger it poses. If you are starving and she says "you're hungry", she could cause something simple and make you find money for food, or she could make someone giving you food for free, but she could also cause a chain of events where you end in jail and get food from there. Or you end dead so "hungry" doesn't apply anymore.