I'm not sure it's creditable that enough credence is being given to these anonymous claims in a private group to be impacting this person's life. Seems more akin to a slapp.
https://www.thedailybeast.com/ugly-battle-over-shtty-media-m...
Some cases were probably well desreved, but anonymity allows easy score settling and revenge.
There's no recourse against an anonymous accusation, there's no way to defend yourself, and there's no way to prove you are innocent.