Is it permissible to give a wedding dress to a non-committed bride, when the questioner knows that her wedding will be mixed and she will wear makeup, and is there any sin upon her for doing so?

It is permissible to lend clothes to a bride, as it involves benevolence and acts of kindness. Al-Bukhari titled a chapter in his Sahih: "Chapter: Lending for the Bride at the Time of Consummation," and he cited evidence under it supporting this, as 'Aisha (may Allah be pleased with her) used to lend her chemise to brides. However, it is not permissible to lend clothes to someone if it is known or highly probable that she will use them for something forbidden, such as displaying her adornment to non-mahram men; this is based on Allah's saying: "And do not cooperate in sin and aggression." Therefore, assisting in disobedience is forbidden.