I'm asking this question for a friend. My friend wanted to pray Namaz but before that he had preformed an impure act. He didn't know at the time but he just preformed the obligatory Wudhu and went on praying his Namaz. He asks if it is wise to just preform Wudhu instead of a entire Ghusl before Namaz and/or fasting.
All Praise to Allah Subhanahu wa Taala and blessings of Him be on Prophet Muhammad peace be upon him His family and companions,
Ghusl becomes compulsory after:
Ejaculation ,Intercourse, Menstruation and Post-natal bleeding.
Hadath-Akbar refers to a state of major ritual impurity. The condition of Hadath-Akbar requires the complete ablution (al-ghusl) before one can pray, circumambulate the Kaaba; touch, carry, or recite the Quran; or stay at the mosque.
So if your friend is in a state of Hadath akbar then he has to do Ghusl and he has to redo that Prayer.Such person cannot pray or Fast just by doing wudhu or tayammum.
Also see this.
Allah Swt and His Messenger knows best.