All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
Your question is about singing and music, which lead to corruption and debauchery. Both singing and music are forbidden by the Quran and the Sunnah.
Proof from the Quran says (what means): {And of the people is he who buys the amusement of speech to mislead [others] from the way of Allaah.} [Quran 31:6] Ibn ‘Abbaas and Ibn Mas‘ood both said, “(The amusement of speech) refers to singing.”
As regards evidence from the Sunnah, Abu Umaamah Al-Baahili said, “The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, forbade selling female singers, buying them, their earnings, and consuming their prices (i.e., spending what one would get for selling them).” [At-Tirmithi, Ibn Maajah, At-Tabaraani, Ibn Abu Ad-Dunya and Ibn Mardawayh] The same Hadeeth was also narrated by Abu Ya‘la on the authority of ‘Ali .
It was also narrated on the authority of ‘Uqbah ibn ‘Aamir that he quoted the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, as saying: “Everything with which man amuses himself is vain except three (things): a man shooting his bow and arrow, training his horse, or playing with his wife. These things are true.” [Ahmad, At-Tirmithi, An-Nasaai‘i, Al-Bayhaqi] [As-Suyooti: Hasan (Sound)]
It was also narrated on the authority of Abu ‘Aamir or Abu Maalik Al-Ash‘ari both, that he quoted the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, as saying: “There will be among my Ummah (nation) people who will permit fornication, silk, alcohol and musical instruments.” [Al-Bukhari]
Because singing and music are forbidden, any contracts that sell anything related to them or any related lease contracts are null and void and thus no one is entitled to any right pursuant to them. It is therefore not right that your friend contacts the original owners of the old song. Also, the money earned from the lawsuit is forbidden for the Muslim to take or consume.
Allaah Knows best.