Is it permissible to cut ties with a friend who speaks and jokes with girls, and to warn others about him?

A man's conversation and joking with women who are not his mahram is a reprehensible act that is impermissible. He must be mindful of Allah and avoid actions that incur His wrath. He should be advised and reminded of Allah gently and kindly. If he repents, then praise be to Allah. If he does not repent, then shun him if you believe that shunning will benefit him. The fundamental principle is the inviolability of Muslims' honor and the obligation to conceal their faults. It is not permissible to inform friends about what is happening unless there is a legitimate, recognized Sharia interest, and the disclosure should be limited to the extent necessary to achieve that interest.