Is sitting with an evildoer considered a minor sin by way of showing familiarity with him, and is this statement based on a legitimate religious proof, and what is the definition of an evildoer here?

What was mentioned, that sitting with the wicked encourages them, is incorrect. Scholars have stated that the book "Tanbih al-Ghafilin" by Al-Samarqandi contains many fabrications. Sitting with wicked people is generally forbidden. In the noble Hadith: "The example of a good companion and a bad companion is like that of the musk seller and the blacksmith's bellows." This indicates the virtue of sitting with the righteous and the prohibition of sitting with evil people and innovators.

It is not permissible to sit with a person while he is committing a sin, except for one who intends to admonish him. If one is unable or unwilling to admonish, then he must avoid him. A Muslim may find it necessary to sit with wicked people out of necessity, provided he admonishes them and does not approve of their actions.

Backbiting is forbidden by the consensus of Muslims, and many scholars have considered it one of the major sins.