If there is an opportunity in the market as we currently seeing now, and an investor wants to take advantage of it, but don't have enough cash at hand to do that, and such an investor has a reasonable amount of money in his reserved funds account, he can can take half of the money from his reserved funds and take opportunity of the dip and buy more stash of bitcoin. Later on he can gradually replace the portion or part of the money he took out from the reserved funds. Provided the investor is not taking out all the money in the reserved funds to accumulate bitcoin it's fine with me.
In such a DIP situation, one should buy as much Bitcoin as possible, but I think it would be more reasonable to set aside funds from the beginning to buy Bitcoin at the DIP price only.
Specific funds should be used for specific purposes, if you keep funds for your emergency situation, then that fund should only be used in emergency situations, spending it on other things can be a bit risky.
However, as you said, if the investor does not have enough funds to buy the DIP, in that case he can invest half of the money from his other funds, but he should replace the fund again as soon as possible.
But, this type of decision should be made only that situations, when the investor's long-term goal is clear, because if the investor's goal is to make short-term profits, then buying a Bitcoin DIP with emergency funds in such a way is never reasonable, because it will be trading, and as a result of this type of activity you will lose your important fund.
Therefore, when making such decisions, the investor must be responsible and maintain a long-term perspective, and also must replace the investment as soon as possible.