I think the best thing you can do is write a post about it and solicit the help of other steemians. Steemit is a global community and when a member of that community is threatened or in trouble the other members of the community must come to his/her side.

The person who got flagged should reach out to other people to equally make posts about what has happened to him/her that way the person that flagged him/her will not be able to hide the truth by flagging everyone.