Is gifting a song about the Virgin Mary, according to the Quran, to a Christian friend considered an act of alliance with disbelievers? And if so, how can I rectify what I have done?

Donating the song to a non-Muslim is permissible and falls under the category of good treatment. There is no objection to donating it as long as it conforms to the Qur'an and is free from polytheism and forbidden things. There is no objection to rejoicing in it as long as it conforms to the Qur'an, and perhaps that will be a cause for her guidance. The love of Sayyida Maryam is a part of faith, and what is prohibited is exaggeration regarding her and Christ, peace be upon him.