What is the ruling on backbiting as a joke among friends, knowing that the one being backbitten does not dislike it?

Gheebah (backbiting) is your mentioning about your brother that which he dislikes. If what you say about him is true, then you have backbitten him; and if it is not in him, then you have slandered him. The Prophet, peace and blessings be upon him, mentioned "that which he dislikes," which implies that if the speech is not disliked by the person being spoken about, then it is not gheebah. This is called "Mafhoom al-Mukhalafah" (the concept of contradiction), and it is an argument accepted by the majority of scholars. Thus, the criterion is what the backbitten person himself dislikes, not what others dislike, because the Prophet, peace and blessings be upon him, said: "your mentioning about your brother that which he dislikes."

However, a servant must be cautious not to expand on this; for he might fall into mocking Allah's creation, or lying, or indecency in speech. And unless there is a legitimate Shariah interest, what is appropriate is to guard one's tongue.