Does a man mixing with a woman in Ramadan and going out with her without an official relationship or the knowledge of her family, and being in seclusion with her, invalidate the fast?

Praise be to Allah, it is confirmed in the Sahih that whoever does not give up false speech and acting upon it, Allah has no need for his fasting. Falsehood includes mixing with women and being alone with them and other prohibitions, and these sins nullify the reward of fasting and reduce its benefits. These sins do not invalidate the fast nor require making up for it unless something that invalidates fasting occurs, like the emission of semen, but they do reduce the reward. Therefore, the Muslim should avoid sins, both minor and major, especially in Ramadan, in obedience to His saying: "Fasting is prescribed for you as it was prescribed for those before you, that you may become righteous."