Edited out
The reason for giving such advice is that there may be times when money may be needed, but at that time the investor is not able to invest in the same way, so he should invest $40 and keep the remaining $10 in his emergency fund so that he can use it in any danger and so that his investment remains consistent even during financial disasters.
But don't you think that before the investor arrived to the conclusion that he/she will invest certain amount in bitcoin through any strategy that such investor adopted, that he has already made provision for emergency funds on their own? In as much as you meant well for such an investor, I think the best approach is to find out from the investor if he has made provision for emergency funds or not before proceeding with such an advice? You have to know the state of things before knowing what to advise the investor. You can't just assume on your own that such investor has not made provision for emergency funds on their own.