Do you have a life long friend that if you heard a rumor about them, an accusation, that you wouldn't believe it because you have spent a life with this person and feel you know them well enough to know a lie when you hear it?
Or the reverse of that. You have a life long friend who one day sits you down and tells you about something that happened to her long ago but that she has never spoken about (or very little) and you are compelled to believe her because your long friendship has given you no reason to doubt her?
So the friend can both not remember the incident personally but believe her friend because there has never been a reason to doubt her friend during their friendship?