If a man and woman prays their salah (namaz) separately (furadah) in one room, is it allowed a man can pray his furadah salah (namaz) after (behind) a woman in the same room and is his salah (namaz) valid

If they are praying at the same time then the woman’s place of sajdah must be behind the place where the man places his knees otherwise both of their namaz is not valid. Alternatively, the man can wait and then pray once the woman is finished.