Is it permissible to provide financial assistance to a poor person for the purpose of preparing his daughter for marriage, knowing that the wedding will include songs and music that contain religious phrases, and that the assistance is designated only for clothing and preparations, and that the music will be brought by the groom, not the poor person?

You should advise the wife's father on the prohibition of music and musical instruments, and the obligation to denounce this to his daughter's husband and prevent him from it, as this is among the forbidden acts that remove the blessing from marriage. Lawful entertainment and beating the tambourine (duff) have been legislated as an alternative to this. As for financial assistance, there is no harm in you providing it as long as you do not know that it will aid in forbidden acts, and you are not affected if the husband brings musical instruments with his own money.