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.
Using the amount you can afford to lose doesn't necessarily mean that you don't have to apply it to Bitcoin investment. You also have to know that after settling your monthly expenses and putting some funds in your emergency fund box, the remaining is what you can afford to lose because you have settled the necessary things first. Every investment is risky and Bitcoin is not excluded, but Bitcoin is less risky and if you can read my previous comments again, you can see where I said, knowing the difference between Bitcoin investment and trading, he could know one with less risk. Moreover, life is full of risk, and you should know that investments are not excluded whether Bitcoin or not. However, Bitcoin is a safe heaven investment if we apply a kind of strategy called the DCA method and I believe you know about that strategy too. The trading pattern is like buying when low and selling when profits are made, and it's risky that way too, and it doesn't give the trader enough time to buy more, because there are times when Bitcoin price is high and the trader will have to wait until Bitcoin reduced to some certain amount before he/she could buy again, but applying DCA method gives you the ability to buy at all time when you have the money, even when Bitcoin is high in price you could buy and not only wait until it reduces before you buy.
Buying at a low price and selling at a high price is one of the concepts and goals in investment. You can buy and hold it for the long term to get the profit you expected when you started investing.
If you have these thoughts on your mind, then you are not ready for the long-term Bitcoin investment. In my opinion, if you are interested in investing in Bitcoin, you don't have to think of the amount you want to make, only pursue the goal and don't settle for the goal. Bitcoin investing is an asset and selling your assets because you have gotten what you want is not a good idea, because the figures you sold today will change to bigger figures in the future. Let's use a house, for example (if you are not dealing in real estate). Building a house is a thing of gradual process, and you might rush the building because it's a physical asset. Then, after building is complete, you sell the house for no reason or for reasons. What's the essence of that? Since Bitcoin investment is not a physical asset, there's no need to sell everything you have gotten (both capital and profits). However, it's only a trader and someone that doesn't know what bitcoin is can only sell his/her assets after making profits. As Bitcoin doesn't end, profits don't stop coming when you are investing for the long term. So my option is that continuous investment is the only goal.