What is the ruling on a father preventing his daughter from befriending her friend, despite their 15-year friendship and the absence of any offense from the friend?

You must end the estrangement between you and your friend if its cause is worldly, because discord and turning away from each other among Muslims, when not for a legitimate religious reason, is forbidden. You should continue to greet her, visit her when she is ill, be kind to her, apologize to her, and try to convince her father of the seriousness of estrangement among Muslims. If she persists in severing ties after that, then you will have absolved yourself of sin, and she will bear it. Many hadiths have been narrated prohibiting estrangement for more than three days, clarifying that the best of those who dispute are those who initiate the greeting, that estrangement for a year is like shedding blood, and that Allah forgives Muslims except those who dispute until they reconcile. Encourage your friend to reconcile, for Allah Almighty says: "No good is there in much of their private conversation, except for whoever enjoins charity or what is just or reconciliation among people," and "Reconciliation is better." And be keen to be kind to her, in compliance with His Almighty saying: "Repel evil with that which is better, and then the one whom between you and him is enmity will become as though he was a devoted friend."