Exactly, most persons don't understand this and some people venture into investment because of influence of friend(s), because they heard someone talking about Bitcoin investment ( how much they have made or how they are going to make profit) etc they allow all these swift them off their feet without even having a second thought about it or check if there friends have other source of income that is making them accumulating without touching or complaining of money to sort out some stuffs.
Some people who immediately believe in Bitcoin investment on the advice of their friends actually do not do anything wrong to themselves, but it would be better for anyone who accepts something new in their life to be willing to find out about the knowledge of it so as not to mislead themselves even though it is a very good thing to follow. In addition, it is also true as you said that people who receive advice from their friends should see where the person gets their income from so that they can also follow the same steps before jumping into investing with all the capital they have.
Because it will always be easier for investors who have other sources of income when they want to invest more focused on something like Bitcoin so that they can continue to maintain it well without being burdened by the daily needs of their lives. So look for as much income as possible and then allocate a few percent of it into investments such as Bitcoin because something like that will make it easier for us to manage investment assets and will also be much more focused when investing.