Learning about it does not make someone fully trust on Bitcoin. People's opinion will only change about Bitcoin being positive when they learn everything about it. When I say everything I mean everything. From the good, to bad, to the worst. Every single details need to be known for person to fully understand what Bitcoin really is and what it can do.

Assuming that your friend knows enough but still don't want to trust Bitcoin, then it is his personal choice. If they are not interested in it then leave it like that. Him joining crypto world won't make any good for you. So why waste time on people who are ignorant? If you are happy with your decision about joining the crypto world, be happy with it. What others do and doesn't, it shouldn't affect your life at all. Be on your own and write your own story. Do not interfere in other people's personal space. And never give financial advice to people even if they are your best friends.