A relation should be used with things: size, shape (other descriptions) or placement. In relation to this room, you wold go down through the north door and right down the hall. In relation to the car, the model is 1/5 size. In relation to you, I am about the same personality. The other terms deal not with comparing physical (or descriptive) attributes, but more often with social interactions. Of course a relationship may or may not be a positive thing, yet the terms using "friend' imply a positive relationship.

Basically, "relation" and "friend" refer to the person involved, and the condition/state of being is known as "relationship" and "friendship." Example: She is my best friend—our friendship started when we were children.

A kind of relation makes a relationship possible. One must have a friend to enjoy friendship.

Relationship and friendship are terms used when the relation or encounter begun while happily sailing over the sea.