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You should have done this earlier or into those times that Bitcoins price isnt moving that much or not really that high in value or not those high prices already. The wrong thing for most people is that on the time that they would be seeing those green candles then this is the moment that they would really be considering that all in buy and not into those times when the market is still red. Well, this is really that a very very common trader or investor behavior specially to those who do lack knowledge and experience on which sooner or later they would really be able to realize that they should have done the opposite.
I agree with you, it was a mistake they shouldn't have made. However, experience and knowledge over time will teach them when is the best time to buy and accumulate and when is the best time to sell. They will never learn without small mistakes even though they can learn from other users' mistakes.
If you are someone whose really that go for long term then any price entry wouldnt really be that much of an issue since you would really be that holding it for long term.
Any price movements wont really be that much a problem since you wont really be selling anytime soon.
Of course, if they have a long-term investment plan then they must be able to ignore price volatility. They can buy at any price, but of course buying when prices are falling is better than when prices are moving up.
After all, prices will never always increase. This means that there is always a decline phase regardless of the percentage and that is the best opportunity for accumulation although additional strategies such as DCA are also worth considering. DCA and accumulation are a great combination in building an investment portfolio, but of course it really depends on the interests of each investor.