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.
What he said about investing the money you can afford to lose is not about trading or gambling with your bitcoin investment, but a step that must be taken to help us have a good bitcoin investment that we will not sell in the short term. This is the problem so many investors are having with bitcoin investments that makes them sell their bitcoin investments so early to survive. When it comes to bitcoin investment, we should not invest our whole money in it, so that we will not even think about selling it one day to survive. We should invest the money we can afford to lose or we can do without for a long term so that we will not find it hard to solve our problems.
Regards to the issue of investing what one can afford to loose which connote money one will not border much to withdraw or sell off has been a bedrock if discussion since the forum is created I believe people who still get confused with statement like this need more to read in most pin post.
To invest on Bitcoin what you can afford to loose is very important this don't signify is gambling but money that will not border you to sell after being invested this don't happen to investor by mistake but by proper planning BTC investment need a good plan and many of the plan has been discussed virtually on daily basis here which includes emergency fund or reserve fund having other sources of income and engaging on stable income source.