Is it permissible to practice masturbation or self-gratification between spouses, with or without intervention, and is it permissible for the wife to do that with her hand for her husband, or vice-versa?

Masturbation (self-gratification) is forbidden, whether a man does it with his hand or a woman does it with her hand. It is not permissible to do it, even if it is in front of one's spouse or vice versa. However, a man masturbating with his wife's hand or vice versa is permissible, as it falls under permissible enjoyment.