Your friend made a mistake, bought BNB by paying 0.0029TC.
For example, if you buy BNB from 0.0019 with all your money and the price drops to 0.0015, you can sell panic.
But if you buy BNB now with 25% of the money and the price drops, you can buy 25% again. Thus, the cost of 1 BNB is reduced to 0.0017. And if the price rises to 0.0019 again, if you sell half of what you buy, you reduce the cost to 0.0015.
Buying a piece is an important step for a trader to always make a profit. If you deposit all your money in one coin at a time, this mean, you bet on the roulette table.
Your goal is to buy BNB for long-term investment or just to find a small profit from the trade? if for a long term investment then I think it's a good thing, but if for trading the way described by this person is good enough, because I often do this way to trade. this method can be utilized but you must still have a strong analysis and a precise calculation in percentage