This is true. It's rarely talked about, but it can really be devastating. A close friend often knows our deepest thoughts, secrets, and vulnerabilities. If they end up turning on you, they can end up really hurting you with all that. Not only that, there can be deep, profound grief over the loss of a longtime close friend.
It's happened to me in the past. I tend to be rather guarded with my friends of today which can have a set of problems as well.