A Human Being's Right to defend themselves againat harm or oppression is not dependant upon their gender, nor their religion
Our LordHis MessengersWe have so much to learn
Every Muslimah is a sister to every Muslimah.Your diligence in defending her honor and reputation should be no different in her absence than it would be in her presence. You can help yourself with this in two ways: Imagine that what was said about her was said about you in her presence. What would you like HER to say in your defense? Then do likewise in defense of her honor. Imagine that she is present behind a curtain and hearing everything that is said. One who is not sincere in Sisterhood is a hypocrite. Sisterhood also includes and demands educating and sincerely advising. Your sister's need for knowledge is no less than her need for food and shelter. Whoever occupies herself with her sister's needs, Allah occupies Himself with her needs. Whoever relieves a Muslim/Muslimah of some hardship, Allah will relieve him/her of one of the hardships of Qiyama. And whoever conceals the fault of a Muslim/Muslimah , Allah will conceal his/her faults on Qiyama." (Bukhari) To fail to come to the defense of her honor is tantamount to "surrendering" her. Protect her as Allah gives you the ability to, even if it be from herself..