In this given scenario, John has two (2) options: either to invest the remaining funds and HODL; or gamble everything with the risks involved.

The decision would highly depend on the immediate need or emergency situation of John if he needs the money right now. Remember that in investing, the results may vary depending on your luck and the situation on the market. Unfortunately, he bought the bitcoins priced when its price skyrocketed ($18,000) but since it has dropped significantly, he can either invest it again OR gamble everything.