Usage patterns
〈人〉を友とするto consider someone a friend; to befriend someone
I decided to make him a friend.
旅の友a travel companion (e.g., a guidebook or item that accompanies one on a trip)
I brought this book along as a travel companion.
良き友を持つto have a good friend
Having a good friend is a treasure in life.
友を選ぶto choose one's friends
Choosing your friends is important.