Is regretting, seeking forgiveness, and praying for the wronged friend sufficient expiation for the sins committed against her, and what must be done to ensure God's forgiveness?

Your error is twofold: informing the young man about your friend's relationships and helping her to get into trouble, whereas the proper course of action would have been to advise her sincerely. The other error is your own commission of the same sin by being alone with the young man and speaking with him. Sincere repentance, with its conditions, expiates all sins.