I've seen this happen many times. Not just to friends, but co-workers and people that I have met along the way.
In one case I had a friend who was hired by a former classmate to a local firm. After a year there they started hiring more programmers company wide. This friend refused to interview people who had graduated from our department or pass along resumes. Instead they started hiring other people from non-engineering departments that they thought they could train to program. This guy is now a C-level and still refuses to hire people that graduated from our department.
I'd wager that it's just a feature of my particular network and people I've come across. As my partner is the opposite, and their network helps each other out.