Does my participation with my friend, who slanders people, become sinful merely by listening to her, despite my repeated attempts to defend the people she talks about?

If your friend harms people with her tongue, she is in great danger, and it is obligatory to advise her and remind her of Allah, to clarify the grave consequences of these actions. The Prophet, peace and blessings be upon him, said about the woman who harms her neighbors with her tongue: "She is in the Fire." Acts of obedience deter one from sins. You have done well by defending the person she slanders, but it is not permissible to sit with her if she persists in slandering people's honor, due to the Almighty's saying: ﴿And when you see those who engage in [offensive] discourse concerning Our verses, then turn away from them until they enter into another conversation.﴾ If shunning her would benefit her, then shun her.