Should one leave an engaged woman due to her father's drug addiction, despite everyone praising him and the girl, and considering the potential impact of this on the fiancé and his children in the future?

If the father of the girl is suspected of drug abuse, the original presumption is his innocence until proven otherwise, according to the Almighty’s saying: “O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin.” If his drug abuse is proven, it does not legally prevent your marriage to the righteous girl. You may even be rewarded for intending to save her from a bad environment. Her righteousness will aid in your religious affairs and the future of your children, and you both may be a cause for reforming her family.