Duck, no one knows what tomorrow brings. Some of us make guesses based on a variety of information: Technical analysis, market sentiment, psychology, economic fundamentals. But in the end, it is still a guess. Some make no guess (e.g. let the Vanguard computers re-balance over time automatically).
One thing your friend can do to calm themselves is to look at historical data. Over 1 year, the SP 500 historically has been up 151% (best) to down 58.1% (worst). Over 5 years 33.3% (per year best) to -13.2%. Over 10 years +19.6% to -5.9%, over 20 years +13.6 to -0.2%, 30 years + 11.2% to +1.9%, and over 40 years +10.3% to +3.2%. So, the longer the time period the more likely the market (as measured by the SP 500 index) will be higher. Even with this, it is not predicting the future, but rather measuring the past, and there is nothing to say that the US equity market in the future will be like the US equity market in the past.
In another thread I posted a link to this post in Boggleheads:
https://www.bogleheads.org/forum/viewtopic.php?f=10&t=79939&newpost=3771802
The point of it is that it is only in times of stress that one gets a real feeling for their true risk tolerance. If your friend is being impacted (mentally, emotionally, physically) by the market turbulence an drop, then there *is* a message here to them about needing to adjust their risk profile.
At the low today, I was down somewhere in the range of 250-275K from the peak. And yet, I can guarantee you that tonight I would have slept just fine even if the market closed on the low. Why? Because I have some cash, savings bonds, CD's, and other fixed instruments (plus income from a pension) to help keep my emotions in check. I also have a philosophy that if I have a roof over my head, food to eat, and decent health then everything else is a bonus.
One other thing. I do think it is sometimes important to just "do something". So if your friend is nervous, sell something. Just don't sell everything. There is nothing wrong with hedging a bet. (I call this making a sacrifice to the market gods.)