Is supplicating against the son of a man who engaged in an illicit relationship with a girl without penetration, where he was the one who solicited her, considered an injustice to him, given his repentance and refusal to return to that relationship?

The forbidden practices that occurred between you constitute a sin that exposes one to Allah's displeasure and wrath. You must offer sincere repentance by severing your relationship with this girl. Her prayers against you will not harm you because she consented to the forbidden acts, and she has no right over you. These practices are metaphorically referred to as zina (fornication/adultery). True zina, which necessitates a hadd (prescribed punishment), is the penetration of the glans or its equivalent into the vagina.