What is the ruling on repentance, and what should I do, as I installed a phone line for a friend, knowing that he would use it for forbidden calls, and he is now indeed using it for that?

The questioner erred by helping his friend install a phone line if he knew it would be used for forbidden calls, due to Allah's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." He must offer sincere repentance and advise his friend. If the friend persists in his error and he is able to disable the line through the telecommunications company, then he should do so. Otherwise, there is no blame upon him after his repentance and advice.