A long time ago my father went to a sorcerer/soothsayer and afterwards followed his advice. Knowing that my father was not religious at that time, what should he do now?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Sorcery is a kind of Kufr (disbelief) and sorcerers and soothsayers are disbelievers. Indeed Allaah The Almighty Says (what means): {It was not Solomon who disbelieved, but the devils disbelieved, teaching people magic.} [Quran 2:102]

It is also known that if a person goes to a sorcerer or a soothsayer and carries out what he orders him to do, this person will have the threat that the Prophet, sallallaahu ‘alayhi wa sallam , mentioned in the Hadeeth that was narrated on the authority of Abu Hurayrah and Al-Hasan applied to him. In the Hadeeth the Prophet, sallallaahu ‘alayhi wa sallam, threatened that whoever visits a soothsayer and believes in what he says is a disbeliever in what was revealed to him, sallallaahu ‘alayhi wa sallam. [Ahmad, Abu Daawood, At-Tirmithi, An-Nasaa’i and Ibn Maajah]

As for your father, he committed a major sin by going to the sorcerer or soothsayer, and whenever a person commits a sin he should repent to Allaah The Almighty to remove the sin. It was narrated on the authority of Anas that the Prophet, sallallaahu ‘alayhi wa sallam, stated that every son of Aadam sins and the best of sinners are those who repent. [At-Tirmithi, Ahmad, Ibn Maajah and Ad-Daarimi]

Thus whoever repents to Allaah The Almighty will be forgiven all his sins, including the sin of going to a sorcerer or a soothsayer.  Allaah The Almighty Says (what means): {O you who have believed, repent to Allaah with sincere repentance. Perhaps your Lord will remove from you your misdeeds and admit you into gardens beneath which rivers flow.} [Quran 66:8]

If your father is still alive, he should ask Allaah The Almighty to forgive him. If he is dead, then you should supplicate to Allaah The Almighty to forgive him and give charity on his behalf.

May Allaah forgive him, for He is the Forgiver and the Ever Merciful.

Allaah Knows best.