What is the ruling regarding someone who had a wet dream during the daytime in Ramadan and thought that it broke his fast, so he ate and drank out of forgetfulness, believing that his fast was invalidated?

Your friend has committed a grave oversight by intentionally breaking his fast during Ramadan without consulting scholars. He must offer sincere repentance to Allah. He must also make up for that day because he is not excused for intentionally breaking his fast due to his negligence in asking and learning. The esteemed scholar Al-Uthaymeen, may Allah have mercy on him, stated that one who eats forgetfully, then believes his fast is invalid and intentionally continues eating, must make up the fast. He considered such a person negligent and obligated to seek knowledge. No expiation is required of him, as expiation is only obligatory for breaking the fast through sexual intercourse, according to the preponderant opinion.