I smelled cigarette smoke smell on my helper after she came back on her off day. Should I confront her? Can I ask her to quit if she admits?

Maybe she met a friend who happened to be a smoker? Give her the benefit of the doubt. You can ask her if you're feeling uneasy. If she admits it, then it's up to you whether you are comfortable with the fact. I wouldn't, if she has to be around my children.