Earlier quoted context omitted.
And it's good they didn't invest on your advice to go long on TSLA. If you invested into The diversified SP500 instead with the same timeline (2015 to today) it would give you a slightly similar or even better return than TSLA that went from 195 to 295 ( and that is IF the current price holds until tomorrow which will most probably not). Never forget opportunity cost while investing!
but the person who buys telsa instead of an index fund is likely anticipating increase volatility and has higher risk tolerance
Yeah we have the benefit of hindsight, but that's what we're talking about. If you're just evaluating past performance starting from any point in 2015, Tesla generated average returns, was pretty uncorrelated (which is great), and was way more volatile than the S&P. That's not thrilling.