I made beatthecouch.com in July and it's basically a game where you try to beat the S&P 500 and buy/sell when you want. Your opponent is a couch. It buys on day one and never sells.
Now over 100K+ games later, the hypothesis stands: it's not wise to try to trade and time the market. Here's the original data from the actual games itself.
Source: the game's own database, every completed game Jul 12 to Aug 28. Tool: Python and matplotlib. Market data: S&P 500 daily total returns 1928 to 2019.
Insider trading is definitely frequently used to unfairly beat the market but in your friends defense, there is a difference between knowledge of current world and market conditions and insider trading.