My mom told me that she wants to try smoking. What should I do?

My mom is usually open with me about such things but for the past few days she was telling me that her friends smoke and in winter that looks cool, she herself said that yes it affects our lungs but so does staying in Delhi. She was continuously bringing this up and i didn't understand why was she saying that again and again. 2 days ago we went to an open air cafe and there to her surprise everyone was smoking while having their food and coffee. My mom told me that even she wants to try one, I gave her the "lecture" that it's wrong and all that stuff but i couldn't lecture her more than that. After her being very persuasive, I got her a very mild cigarette and gave it to her in the cafe, she obviously coughed in the initial puffs but then got used to it. She told me to not my dad about it. Last night she again asked me to accompany her to that cafe and we went there she had a cigarette and we came back. I don't know if it's becoming habitual but she thinks that it looks cool and doing it once in a while won't hurt. I myself have smoked 2-3 times in my life but I don't know how to stop my mom before it becomes too much. I know that I'm nobody to lecture her and she's an adult but since I care about her I'll try my best. What should I do?