How can one repent to Allah for the sin of causing another person to persist in disobedience, and what are the pieces of advice for repentance?

If you repent to Allah with sincere repentance that fulfills its conditions, then your repentance is sound and accepted, and you return as one who has committed no sin; for the Prophet, peace and blessings be upon him, said: "The one who repents from sin is like one who has no sin."

Your companion's persistence in sin does not harm you as long as you have repented and advised your companion to repent.

Increase in supplication and seeking refuge with Allah, adhere to His remembrance, accompany righteous people, strive to maintain the obligatory prayers and increase in supererogatory acts, and distance yourself from every cause that invites you to disobedience.