If your friend makes a profit by investing in your words he will be happy. But if there is a big loss, it can have a negative impact on your friendship. So always clearly remind him of the financial risks when giving advice. Apart from these encourage him to study what bitcoin is how it works and what its future holds. Let him know that you are not an expert and your advice is only based on personal experience.
How about also making sure or trying to also learn about your friends and how they react to things, not just about your telling them the basic risk and the warning associated are they the type who truly listen and learn, or they are the type that just pick what suits them and leaves everything for you.
We have some types of people who, no matter how you try to warn them about something, as long as there is something beneficial that could come from it, and it's from someone they know they don't care about the risk they want to jump into investing once you mention it. And even after warning them clearly, they are still the type who is not bold enough to take responsibility for their own action and always want to shift blame, such type of person is not who one should recommend any form of investment for.