If someone is cash-heavy right now and is wondering if they should buy in, I would probably suggest dollar-cost averaging, perhaps with monthly buy-ins while the current craziness is going on (assuming someone is buying into things where there are no transaction fees).
So if someone has $100k to invest, maybe they buy $10k every month of their favorite index fund over the course of the next 10 months. If prices continue to drop, some of that money will buy at lower prices, but if they rebound over the next month or two, at least some of the money will buy at the (current) low price.
Then there's also the possibility of a dead-cat bounce somewhere in all this later on. Even if it looks like prices are going back up, they could drop again, and -- again -- there's still more cash to put in at various prices.
So if someone has $100k to invest, maybe they buy $10k every month of their favorite index fund over the course of the next 10 months. If prices continue to drop, some of that money will buy at lower prices, but if they rebound over the next month or two, at least some of the money will buy at the (current) low price.
Then there's also the possibility of a dead-cat bounce somewhere in all this later on. Even if it looks like prices are going back up, they could drop again, and -- again -- there's still more cash to put in at various prices.