If I'm not mistaken your friend could be a beginner because that's what most beginners who just started accumulating Bitcoin normally do..
I doubt that it us only beginners that does this, from what I understand, it is the mindset that they used to venture into bitcoin investment that will control your emotion and thoughts. There are still some people that are old in bitcoin that are also carrying out such practice, because they never have any plan to invest for a long period of time and also to have a bitcoin target and how to achieve it.
However, sometimes it works for them and they feel that they are smart but not knowing that they were only lucky to sell at the right time and the price of bitcoin dips to favor them, but most times they make wrong decisions and sell thinking that the price will dip but only for the price to keep pumping and they will regret their actions because they will not be able to buy back till they miss out.
This is why a newbie should have a long-term goal mindset with a bitcoin target, so that s/he can stay focus on accumulating with regular DCA at any time either in the bull or dips to grow his/her bitcoin portfolio. With this, you will never think of selling until you have reached your bitcoin target or very close to your bitcoin target, as long as he has made available emergency funds and reserve funds down.
Why do they panic, because they put in large amounts of money and they sell quickly because they are not yet strong enough to accept the reality that prices fall quickly. That's the big difference when they put in big money but don't have the strength to hold it any longer. In investing, the money we use is of course money that we do not use for our living needs, so we will be ready to accept whatever risks occur in our investment journey. For those who target fast profits by buying instantly or more aggressively with all the money they have, of course this will be a big risk if they are not ready to see market changes so quickly.
In this thread in a JJG post where JJG says that for beginners it is better to start investing with a smaller value with dca like $10 and can increase it slowly, this is of course to strengthen our mentality in our investment journey. We can't climb the ladder by going through 10 stages because we need to move one step at a time and that's a good strategy for beginners.
This is the problem of most investors they end up not making profit due to their high expectation, the strategy of buying with little amount for beginners is a great idea but not for only beginniners, a lot of investors who claim they know what they're doing and end up with loss, the ones with low income can adopt this method, for example a salary earner with little income after receiving monthly salary it's best to make a budget, bitcoin investment should be added to the list every month 10% or 15% use in accumulating bitcoin, if investors with little income can follow this process every month the investor will be stress free and can settle other financial problems rather than using a huge amount in accumulating, I will like to observe investors like this I believe they will always want to check the price almost everyday and when it doesn't get to their expectation they end up selling without a target, in long term investment bitcoin is the best but what most people fail to understand invest the little you can afford.
From my observation not all investors will end up making profit for example the ones that never get satisfied with their investment they always want more using the money meant for food stuffs etc to accumulate more they will always end up selling without a target to settle family needs, the hasty and confused ones moving from different coin for quick profit forgetting investment like this needs consistency and patient.