How to deal with defamation in a RP community?

Hello. I am posting this in behalf of a friend who has been struggling with a situation that has been frightening to follow. I have his consent to post this as he is naturally affected by the situation and worries that he would share more than is relevant.

My friend is active in a certain racial RP community as he loves to explore the race's lore and cultural themes, and has been a member of that community for some years now. He had some kind of a fallout with a guild master due to being bullied by one of the members and as he had been committed to the guild, he did not want to throw it all away just because one member was trying to influence others against him.

This bullying was allegedly due to an IC disagreement taken into OOC and my friend was getting overwhelmed by it. He turned to the guild's officer team for help, but he was met with gaslighting and dismissiveness. The GM is also very influential in this community at large and within it OOC connections and reputation mean everything, if you are hoping to do more than walk-up RP.

After the fallout, the GM has been going out of their way to convince other guild leaders that my friend is a heinous roleplayer. A lot of mutual associates have believed them, but a handful of people who know him better, are seeing through the false claims and those with a grain of truth. I do not know why the GM has reacted this way but it is disproportionate to the original issue.

The really frightening part is that someone has allegedly used AI to generate "evidence" in order to make the GMs and bully's claims more convincing. I have not seen this, but it was relayed to me by an insider that they were joking about it over voice chats.

How can this be combated? Not only in the current situation, but if we were to start a guild of our own and build a new, more safe social circle, how can we make fair calls or trust screenshots as evidence, if it can be forged with AI?