When I say that a key element of a friendship is witnessing I imagine most people will think of being there for someone on their journey to knowing God. But I think the Bible calls us to more than that.
In Matthew 28 it doesn't ask us to convert all nations, it asks us to make disciples of all nations. I believe there is a huge difference between someone who says they are a Christian, they believe in God and somebody is a Follower of Jesus. To sum it up, some call themselves Christians and others are called Christians by others (the Disciples were named Christians by outsiders).
One of the biggest blessings I have had is people in my life who have been there for me in hard times, shared verses with me and asked after me. There have been people in my life who have seen me grown in faith and as a person. I have a close friend who has gone through a massive change in her life recently and it has been a joy to be in touch with her and see God at work in her life.
In the Bible Jesus did it with the disciples, Elijah with Elisha, Paul with countless Churches and so on.
I think we are called to do that with others, to see where they are in live, to encourage them, to watch them grow.
