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.
That is not about gambling or trading.. and including if you are investing into bitcoin 4-10 years or longer, you also would not be expecting to need any of your money that you invest into bitcoin in the short term, and you are also not guaranteed to get your money back in the long term either, but surely if you are invested into bitcoin 4-10 years or longer, you will likely see the extent to which the value of your investment into bitcoin ends up going up, down or sideways, and you would be able to make further plans based on how your bitcoin value is performing as you might continue to invest into it or maybe you go into some other status, such as merely maintaining it or liquidating it... but those would be down the road considerations, a tend the value is not guaranteed to be positive or beyond zero, but surely we already likely realize that there are various scenarios that are also possible that bitcoin prices (value) could be multiples and/or magnitudes higher than it is now, even 1,000s of times, but none of that is guaranteed, but instead there are various possible scenarios that are still to be seen in terms of how they might end up playing out in the actual future versus our speculation about how the future might end up being.