My wife has been harassing a woman that I am friends with

I have been with my wife for 3 years. We got married a few months ago. I have been close with this woman for 17 years. We kind of grew up together. She's basically my sister. I have always let anyone I've dated know that this woman is my sister and she's not going anywhere. Some were not okay with that and we partied ways. She is actually getting married next month and I'm her man of honor. We live in different states and haven't seen each other in person for about almost 2 years. We talk on the phone and text at times like I do with all my friends but we all have our own separate lives. Back in January she started getting harassed online by this person who went my GG. Back in high school my friend developed a relationship with one of our teachers when she was 16. It became abusive. He raped her. She got harassed in high school over it but it died down. This GG person was commenting and twitting at her that she was not raped and she's too ugly to rape. She would block this person and they would make a new account sending her really bad texts saying she should have been killed. It's been going on a long time and she's blocked over 50 accounts from this person. I had my wife's phone to call mine and it gets a Facebook message from a mutual friend cussing her out. I click on it and it is one of the GG Facebooks. I go on her Facebook and she is logged into GGs account. And the awful texts are there. She's the one who has been doing this and I have no idea how to handle this. I'm autistic so I tend to handle situations wrong anyway but I have no idea how to confront her. This is such a mean thing to do.