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.
The context of the OP's thread isn't correct in the sense that, a person who learned something can't be said to not have interest in it. I mean nobody forced him to learn bitcoin. So, at that time he was interested in bitcoin. But, to some extent. He did his research, and wasn't satisfied enough to invest into bitcoin. Yet, such a person has invested time, also, in learning bitcoin. Which isn't bad at all. Not everyone would practice what they've read. Hence, making your opinion very correct. His friend's decision should be respected at all cost. What if the going gets tough as usual in bitcoin, and Op's friend lose his money. Wouldn't he feel bad about his friend, whose consistent talk of bitcoin convinced him to invest money in bitcoin, while he had no such intentions of doing such thing. Additionally, personally, people always cut friends that wants them to do what they're into at all cost. Friendship should be respected. I'm not saying Op is forcing his friend to join bitcoin world. But, it's slightly close to that.