What is the ruling on speaking with and jesting with someone who is committing sins while they are doing so, such as someone who is watching immodestly dressed women on his phone, knowing that one is not participating in the sin? And does the ruling differ if the perpetrator considers his action permissible, while I consider it forbidden?

One must forbid whoever commits an evil that is unanimously considered unlawful. If he does not desist, it is recommended to boycott him if that will lead to his rectification. As for being at ease with him while he is committing the evil, it is like condoning it, which is impermissible, as stated in the verses and hadiths that condemn the Children of Israel for their failure to forbid evil. As for matters of disagreement, there is no obligation to condemn them, and there is no harm in speaking to the one who performs them or sitting with him, as long as he believes it to be permissible.