In some cases where you have exhausted your discretionary income for bitcoin investment it's not bad if an investor uses little of his emergency fund to purchase bitcoin and still make a replacement back to your emergency fund when things are alright but emergency fund shouldn't serve the purpose of discretionary income using your emergency in a one time purchase of bitcoin doesn't give you a bad method to invest in bitcoin.
Investing with emergency fund and gambling are the same thing. Because when a person gambles, there is no guarantee that he will not lose his money or make a profit. Just like you have created your emergency fund to protect your investment, if you invest with emergency fund then there is no guarantee that you will not face any kind of financial crisis. If you invest with your emergency fund then how will you deal with unexpected financial disasters. Anything can happen at any time we can never predict the future so it will never be the right decision for us to invest with emergency fund.
If you see a fall in the market and you want to buy aggressively you have money but you do not have it in your hand. You will get that money in your hand after some time or some days. In this case you can invest by taking a loan from someone if you want. But you have to think in advance whether you can repay the loan or not, then you can invest by taking a loan