WIBTA for telling my coworkers gf she is being cheated on?

Cutting right to the chase. My coworker (24M) is cheating on his girlfriend of at least 5 years (24F). I know who she is because of social media but we have never met. I know he is cheating because he has been inappropriately messaging another one of my colleagues (we will call her chelsea) on a fake instagram. Sending inappropriate photos (if you know what I mean) and s*xting (or trying to). He lives in his girlfriend’s parents house with her!!!!

Since becoming aware of that he was in a long term relationship, Chelsea has tried to keep things to a minimum. I don’t know why she hasn’t blocked him but he will still send photos in our work bathroom and when his gf is away. Even inviting Chelsea over to their place for “a night she won’t forget”.

I have seen some of the messages he has sent Chelsea over instagram, and I even found his fake account he uses. But most of the photos are being sent over his fake Snapchat.

Him and I work together every day, he started out our working relationship by making sexual jokes with me and others (we work a corporate job). When I asked him to stop and told our supervisor he got in trouble (obviously) and now we barely speak unless it work related, and he’s quite rude to me when we do speak.

I have seen him flirt with other female colleagues, hug them and he spends most of the day messaging other women on Microsoft Teams.

For some reason, Chelsea doesn’t want to report him to HR for sending pics in a work bathroom on work hours. Maybe she is scared, but i offered to go with her when and if she chose to report it.

His gf and him just went on a romantic vacation and they both shared posts talking about how blessed they were to have each other. This made me sick and made me feel like his innocent girlfriend, deserved to know that she is being cheated on and disrespected. If I were in her shoes, i would want to know.

I have an instagram account I haven’t used since high school, and part of me wants to change the name on it, archive the posts, make it private and send her an anonymous message saying “If you are the girlfriend of insert douche name here, you need to check his phone. Here is the fake account he has been messaging other women on. You deserve to be respected and I know you are not right now”

WIBTA if I do this??? He shouldn’t be able to figure out it’s me as I will use a fake name and have no profile picture but the account looks real. He also doesn’t know that I know what he’s done/doing.