That volatility that scares newbie investors are an opportunity for those who can maximize it. Volatility is the main reason why many investors changed their lives either in a positive way or in a negative way. For me volatility where every newbie scares it the most is the one that can makes us profitable.

Bitcoin can fluctuate rapidly yes, but that's where the profit is (or losses if the opposite). If your friend is an expert in trading (where you didn't stated if he is or not), then he CAN solely depend on Bitcoin, and if he's a trader, for sure he will not focus on it alone, but he will also trade on other altcoins as well. Putting all on his resources into Bitcoin is risky indeed, but not prohibited as long as he knows the risks that he's going to. He's a businessman like you said so if he fails in investing into crypto, he can just go back into selling since he knows how to sell already.

Overall, investing into Bitcoin is risky, but not as risky compared to altcoins, and in terms of impacting your financial stability, Bitcoin isn't a type of investment where if you go all-in, you will lose all of your money in an instant just like a scam coin or project. For me, as long as your friend can make some money not only in Bitcoin, but in crypto in general then he's all good.