What is the ruling on sitting with someone who curses Allah and His Messenger when one is able to condemn such wrongdoing? Is this sitting considered disbelief and cursing of Allah?

It is not permissible to sit with someone who curses Allah and His Messenger, peace and blessings be upon him, without denouncing it. Rather, it is obligatory to denounce him and part ways with him if he insists. Whoever sits with him without denouncing or parting ways is sinful and disobedient, but his sin does not reach the level of disbelief that expels one from the fold of Islam, as long as he dislikes the cursing and is not pleased with it.