Your friend is right, as bitcoin is volatile, there is a chance that it can go down at any moment. Maybe your might mindset is wrong though, the price can't really go on parabolic, no asset in the market can sustain that tremendous growth and sooner or later it will need to go on a correction specially if it is on the overbought region.
There is no assurance, those kind of discussions have been talk about for many years, just a few weeks ago, I also had this some kind of debate with a friend, and the word "assurance" and "risk" are involved. So it's really hard to convince someone because we don't have total control of what is going to happen in the market next.
100 percent agree and true with @TravelMug bitcoin is "very" volatile especially with current prices after all time high will be major correction and then up again
but in my honest opinion not just bitcoin all investment is risk
