What can I do to please my Lord, and how can I convince my friend of our shared responsibility for our sin? Am I the wronged or the wrongdoer, knowing that my sin troubles me and my friend holds me entirely responsible?

Satan is the enemy of man and seeks to mislead him. What happened between you two, your actions, was due to his misleading. Therefore, you both must offer sincere repentance, for Allah accepts the repentance of His servants, and His mercy is vast. Each of you is at fault, and there should be no mutual blame or accusation. Rather, you must both strive for goodness in the future and be wary of repeating the sin. You should turn to what benefits you in your religious and worldly affairs, and associate with the righteous and good people.