All of our friends are imaginary, as is everything else we experience. It is all created by the mind. Even the character we think of as "me" is something the mind creates. When we are young I expect the mind uses these characters to explore a reality that is relatively new to it. As we get older perhaps we integrate these characters into a "self", or else they come and lock us up. Anyway, I expect this friend did what it was intended to do and the mind moved on. It is the same with all forms, they are characters and props in a story. Once we experience the story and get the moral, the rest becomes less important and pushed out of our awareness. There are many people, places, and events I expect I have completely forgotten about over my life time. Occasionally, something will make a connection and they come back. One thing though, what comes back is not a recording of what was, but more like a recreation, assembled from faded pieces of memory with the gaps filled in by the mind. Similar plot and moral, but the setting, props, and characters are improvised on the fly, kind of like eye witness testimony.