I had the opposite problem during a brief stint as "Editor" of a pub, here: the owner sided with a plagiarist. I had the proof. I'd done the legwork.

But frankly, this is exactly why ONLY the copyright owner can legally assert a copyright violation claim. A publication can certainly choose not to publish anything they don't want, but an ACCUSATION of plagiarism is inappropriate. Call it problematic, not sufficiently unique, etc. Say "I have concerns that this may not be the work of the contributor." I will usually make an effort to contact the person I THINK is the copyright owner; it's a courtesy that I appreciate, as a writer. But I did that, once, with a poet and they proved to me that they were posting on two sites - one with a pen name (I emailed them on one contact email address and asked them to reply to me via the other, so I know it was, in fact, the same author. And they were not mad at me - they appreciated my looking out for them.)