Proving Bitcoin is superior to altcoin is simple, just show your friend the market cap, I think that should be enough. However, if your friend insist on buying some altcoins then you don't need to try real hard to prevent it. Bitcoin is definitely a lot safer than altcoins, but buying small amount of altcoin for diversification on a project that he truly believe is not all bad, the only thing you should do is  warn your friend that with altcoin he should be ready to lose all of his money instantly, overnight, if the project turns out to be a failure or the team actually is a bunch of scammesr who finally decided to rug pull. My point is that 'his money his choice' you shouldn't feel worry too much.