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?

Not sure if this is within your control. Is her off day and smoking is not illegal per say. I personally suggest you ask if she smokes on her off days. If she is doing so, just set the rules that she can't smoke on normal days and she has to bath after reaching home from off days.