I don't think it's a "golden rule" to not sell at a loss. But in his situation as a new user, buying actual bitcoin with it's history and growth potential, selling would be a bad call statistically.
That's not how new users think unfortunately. People just buy because the price is going up and they think it will continue to do so. The moment Bitcoin starts to plummet they realize that they made the wrong decision and sell as result.
It technically doesn't even matter what it is that they buy, they are here solely for the bullish market action and are gone when the price is going down. They have no clue about the fundamentals that makes hodlers hodl and buy dips.
Many of these people have been investing in penny stocks, pot stocks, and other means of investing that are highly volatile/speculative. If people understood the power that Bitcoin has, it would be well over the $100k mark right now.