he has 50k on the account, he can pay the platform fees. if the returns minus fees are higher than stock returns sans fees, it is game. and right now, it might as well be.
Bitcoin required a couple dollars per transaction, not to long ago.
You are overestimating his risk, right now, for stocks.
The current setup isn't likely to lose him much money. Throwing darts to pick your stocks will cause your stock portfolio to be surprisingly diversified.
Random stock pick is actually a not crazy investment strategy that is likely to get you the same amount of return as putting your money in an index fund.
Transaction fees in cryto, on the other hand, would cause him to lose 10k a month.
The only reason he is getting away with doing this for stocks is that Robin hood doesn't charge any per trade transaction costs.
Day trading cryto would kill him in trading fees.