Based on the comments on this post, so far my toolkit consists of: leave, report the offender, and learn to protect myself.
I am not satisfied with these answers.
I am asking for ways to show support to a victim of online bullying and harassment.
I will continue on my search to find the answers I am looking for. But I will not be satisfied until the people who have suffered feel safe to share their real, lived experiences, and can freely offer their insights without fear.