Said wiki has been recently adopted by an editor who had to be blocked from the first wiki due to a lot of issues, so probably his reaction would be to block me if I try to contact him or remove the plagiarized content myself.
Is there a way to officially warn him or to involve the FANDOM staff to avoid both the plagiarism and his retaliation?
All content on FANDOM is licensed by CC-BY-SA, which means that other wikis can use a different wiki's content if they attribute it. If they do not state they used your wiki's content, then it means they are breaking the license's conditions. See this page for more info on how FANDOM content is licensed.
If I were you, I would consider doing one of the following with the previous information in mind:
Leave a civil notice here on Community Central on their message wall to amend their wiki content so that it suits the CC-BY-SA license;
Contact FANDOM with the relevant links to each wiki, asking them to step in. They will likely do the same as you would, though obviously they would be carrying more leverage.