If you pick one or two long-term equity funds, he should be fine. Generally higher risk is rewarded with higher return (leaving purely speculative plays out of this equation for now). When you look at a 10-year horizon, then most of the volatility isn’t as important since there is zero need for liquidity now… The big problem with mutual funds is the fees (but Vanguard’s are low) and year-end taxable distributions (based on other people’s trading) – ETFs have low (or lower fees) and don’t provide the year-end capital gains distributions… If he’s just starting out, he won’t be in a real high tax bracket for a few years.
ALL THAT SAID, despite his age, your goals for him are kind of like a 50-year old person looking to retire --> but even more extreme. Rather than needing the money over a 25+ year retirement period, he needs it (almost) all at once in 10 years time. So you’d want to pick your funds now, and put them on a “glide path” with a 10-year horizon. To keep math really simple, that could be diversifying 10% out of the portfolio each year so it’s fully short-term bond/cash in 10 years time. There are an infinite number of variations to this, depending on how much risk/reward he(or you) are willing to take. For example, diversifying 5%/year would have him keep 50% in the market even at the end… or he could diversify 10-20%/year starting in year 6, which will LIKELY provide a better return, but with more volatility.
A multi-year diversification would also help mitigate the big bang of taxes in the final year as the capital gains will be recognized over multiple years.
I’m also not a RIA, so my views are my own (but I might know a little bit about retirement planning).