Up-too-late metaphysical question: Is it possible to invest what you can't afford to lose? Or does the act of investing something mean, by nature, that we can afford to "lose" it?
On a more serious note: If you are "invested" in bitcoin - in its meaning, its value, its possibility, then you cannot lose. Even if the value of your investment goes down, the act of investing in something meaningful is in fact a win.
Yeah, the kid spending all his student loans on bitcoins was more than he could afford to lose. If it tanked he would have gone bankrupt. But since he grew up really poor and his parents never had anything, he figured, "So what?" He took a big risk and got lucky.
If anyone on here took out a second mortgage on your house to buy bitcoins, then yeah, you'd lose your house if it tanks. That would fall under most people's idea of "can't afford".
If someone took out 0% credit card loans to buy bitcoin but really couldn't pay them back without bitcoin succeeding, then yeah, that also falls under "can't afford", because your credit score will be jacked up for years.