What should we do about a preacher who praises an infidel country that is hostile to Sunnis, and slanders good Arab countries, all from the pulpit?

If the preacher errs due to his ignorance of reality, then he should be advised and his error clarified to him. If he responds, then all praise is due to Allah. But if he insists on innovation (bid'ah) and animosity towards the Sunnah, then another preacher from among the people of the Sunnah should be chosen if possible. If that is not possible, then pray behind him, and there is no harm, for the Friday prayer is valid behind every righteous and wicked person, while warning against his innovation as much as possible.