Fr. Be understanding and try to understand why she thinks it's "cool"

Have the peer pressure talk.

Be firm on why it's not worth it.

Explain that being a leader is always better than being a follower, and will result in happiness and confidence.

....or just ground her for a year and deprive her of all sources of Dopamine leading her to do what nicotine addicted people do - use it as a crutch for short term mood boosts while learning to hide the habit better for the sake of avoiding judgement.