When you have a friend like that you don’t have to worry about anything being repeated. You can feel completely free and unafraid. Words that are unrefined or negative are lovingly taken and put through a filter.
I can’t say that is true of everyone I know. Sometimes words have to be guarded because they will be repeated or distorted. These are not true friends and cannot be trusted.
I think that the primary difference between a friend and anybody else is that your friend knows you from the inside out. Acquaintenances only know you from the outside and can never be expected to know you any better. This tremendously limits their ability to relate to you in any meaningful way.