Does one incur sin if they give their friend money to help him with his marriage, knowing that this money will be used in a wedding ceremony that includes unlawful songs?

Easing the burden on those in hardship and lending to a Muslim are among the virtuous deeds and means of salvation. For whoever relieves a Muslim of a distress, Allah will relieve him of a distress on the Day of Resurrection. And a loan is akin to half a charity.

However, if you know that the loan will be used for a sinful act, do not lend, so as not to assist in sin, as Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression."

But if he is in need to complete the permissible requirements for marriage, and you are certain that he will not spend it on something forbidden, then there is no harm in lending to him; rather, it is recommended. And there is no sin upon you thereafter if he commits a sin with his own money or with someone else's money. It is advisable for you to counsel him and admonish him to abandon reprehensible acts such as prohibited music.