Encouragement can do wonders!
I believe that encouraging your friend will do more than "naysaying". Maybe, just maybe, they will finish the ride and it will change their life because they accomplished a goal; maybe they will ride only half way and it will change their life because they worked toward a goal; and maybe they will plan for the ride and it will change their life because they now have new goals and dreams yet to accomplish. To me, cycling is more than about riding the bike, it is life-altering, and as cyclists, we need to support each other by help and encouragement.
A personal story: Way back when, over 20 years ago, I was in college and owned a tour bike. I never really rode it, except sporadically. In fact, during my last year of school it was stored in my parent's house 100's of miles away! Right before graduation a friend of mine, who grew-up in New Orleans, asked me if I would ride my bike with him, from Georgia, to visit his mom in New Orleans --he had no other way to get home but with his bike and did not want to ride alone. I said "yes". So, after graduation I picked-up my dusty bike and rode it, full gear and no training, to New Orleans. It took a week and we rode through 5 states to get there. NOT saying is a wise idea, but I did it -- mind over matter, or just the stupidity of youth, but I did it. I will never forget the experience. It changed my life -- not immediately, but long-term, and in so many many ways!!! So, you JUST NEVER KNOW ... a bike can do wonderful things to people, especially when they least expect it!