All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
First of all, may Allah reward you for your keenness to help your relative repent and quit smoking. We ask Allah to make this happen.
We advise you to supplicate Allah for him as much as possible as everything is in His Hands and everything is easy for Him.
Allah says (what means): {And your Lord says, “Call upon Me; I will respond to you.” Indeed, those who disdain My worship will enter Hell [rendered] contemptible.} [Quran 40:60]
What you want to do in order to achieve that noble purpose is a kind of lying, which is forbidden and it is one of the grave, major sins. It is not permissible to resort to it unless there is no other means to achieve the purpose. Moreover, you do not need to do so here. There are many means issued by health authorities and realistic stories that can benefit you in this regard.
For more benefit on the means to help quit smoking, please refer to fatwa 88467.
Allah knows best.