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.
Anything that pertains to investing requires money which you can afford to lose (as commonly used), it is never a story of it being trading or gambling. You cannot be too excited or let optimistism kill you not to understand the fact that, there is possibilities of loss while investing be it in Bitcoin market or any. We all understand the value of Bitcoin but that does not mean we should adhere to aome guidelines and thus invest everything wether it gonna yield or not. Every Investment we have ever made has risk, I don't get were you are having such assumption from;
This is a simple stuff that you should understand before now, no need to overstressed it.
Let us be optimistic, same time never undermine any possibility in a financial market.