Base and corrupt thoughts are not friendly, thus an individual does not feel that way within themselves. Since corrupt people think all others are like themselves, or seek out other corrupt people, they are not friendly with others as well as themselves. Finally, corrupt people think that harmful practices are normal. A prostitute thinks that selling sex is normal. Thieves think stealing is alright. If these practices are morally acceptable to an individual, then when these actions are done to them it is assumed to be normal. Thus a base person is no friend to themselves. A true friend will stop harmful actions, but the corrupt person does not know right and wrong.
True friends help one another out. If a person is corrupt, the goal is to only help oneself. It is abnormal not to help another for healthy individuals, but normal for corrupt people. How people treat others is what becomes acceptable for them. Even morally bankrupt individuals have a compass telling them right from wrong. This compass in corrupt individuals affirms negative behavior. The negative behavior is normal to these people. If they could treat themselves better, treating other people better would not be a problem. That is what Aristotle was trying to