She’s addicted so she can’t just stop like that. She would need to learn new coping strategies, understand the driving force behind why she smokes, unlearn all habits and behaviours and use her desire to wanting to stop with getting help. Professional help. She’s lying mostly because she is carrying shame, and most likely to please you. Try understanding from an addicts perspective as it’s not so easy to just stop. Maybe she should have come clean initially but to her that’s probably not going to go down so well on a first date etc.
You have a choice to leave and if you chose that then that’s ok but as long as you don’t add pressure by giving ultimatums – that doesn’t work and possibly her smoking will increase.
First off she could try to reduce the amount she smokes, see her GP for help with mood and sleep because that will be affected and then get some help from a drugs worker with the rest.
Best of luck to you both