It is very simple because I trade many time or withdraw many time and I get money in my bank so I think it is very simple I will give them bank proof that time they invest it. But I will tell him it is risky if they have extra money then join bitcoin or trade in it.
A normal who don't know about crypto currencies will not be convinced easily so we can make some effort to teach them because if bitcoin grows its value also grow up that's why I'm saying this.