If they know what they are doing and if they are aware of the risk of investing in bitcoin by buying it then there is nothing to worry about. Even though investing in bitcoin is risky it is still a great choice as an investment because if you will just hold it for a long time then you will surely make money.
There is risk in every investment and everyone should be ready for that risk. As long as people can understand this, they can decide to do what is pleasing to them.
After all, someone decided to sell all he has recently to buy bitcoin and saw it like buying into his future believing he will achieve hugely from it. He is using what he has to trade to live through each day and looking at what he sold, that is not going to be a small amount of money. So it depends on each individual and the level of risk they are willing to take.
If were not a day trader, them holding it will be the best solution since it can increase more, biy when it dumps a little since it will not drop anymore to $4k or to $3k, it will keep to inceease, so buy when you can.