Investing a decent amount is also another way to say invest with an amount you can afford to lose. Even if one is responsible or irresponsible, he/she should know that it's very important to invest with an amount that they can always afford to risk. Everyone has an amount that they can afford to lose, but most people don't know.
I don't know if I'm getting you right but why do i feel that your perception on Bitcoin investment is for trading because when you advised him to use the only money he can afford to lose, perhaps indirectly you are saying that there is a higher chance for him to lose his money Bitcoin or in other words referring to gambling investment, however in as much as nothing is guaranteed when it comes to investment but with Bitcoin success is certain if you are ready to hold, so actually the risk you are talking about should be actually dependent on the kind of investment you are talking about because if your pattern is for a long term holding I wouldn't use the word risk because it seems that you don't really understand or know the potential of Bitcoin you are investing on.
Any money you invest into bitcoin should come from disposable income.. so by definition it is extra money and/or money that you can afford to lose.
Your bitcoin investment could go to zero, so any money you invest could be lost 100%, and you should be prepared for that possibility with any money you invest into bitcoin.
Of course, every investor should make a decision to invest in Bitcoin based on holistic considerations. Overall consideration here is the possibility of high profit or investment loss. Consideration of disposable income as capital and the expectation of potential profit associated with it certainly helps to consolidate the consistency of each buying. Future determination of hard-earned income and the decision to hoard Bitcoin is definitely going to be positive as the system has often cheated one following the historical footsteps of the local currency investment journey.
An investment decision with potential returns like Bitcoin and its past price history is certainly a beacon of hope for small investors like us. Although you always say it can be high and it can also go to zero. Your whereabouts must be reasonable because none of us are sure where its value will go. But I think behind the negative attitude we must keep the positive influence which can encourage us to accumulate more Bitcoins. Long-term trends in holding and holding during bullish and overbought each dips.