You can't, unless they want to. I've had people ask and try, and refuse to even consider it, try and give up at the first or second problem, it always comes down to the same thing, the other persons true desire to quit or vape or not. It depends on their wish to not anything you do, nothing can change that. Try to keep in mind vaping takes more thought and work then smoking a cigarette unless the person wants it its not going to work for that reason even if you get them to try it and many smokers, myself included at one time, resent when you keep offering to help them try an alternative or to quit, no matter what your intentions are if they love to smoke the push to get them to change only comes off as an attempt to change them, and a refusal to accept that they are adults and may choose to smoke, risk or not, if they want to, it feels patronizing. Just do what you do and if their interested they will ask you about it, in which case help them get set up with an ego or something similar that won't give them a ton of problems to start, answer their questions, help them figuring out what works for them, that is all you can do, from there the rest is up to them.