I would add to DoubleAA's answer, that although the MY community is "not in the business of deciding who is a legitimate rabbi", the poster may want to edit the post so it doesn't say/imply anything unintended. From the poster's perspective, this may no longer be a legitimate source at all.

I think a comment would be appropriate.

"[E]ncouraging the author to rewrite the post" (from the question) might be a little strong; perhaps something like "You may want to consider this in your post.", and maybe a link to this question.

I would add to DoubleAA's answer, that although the MY community is "not in the business of deciding who is a legitimate rabbi", the poster may want to edit the post so it doesn't say/imply anything unintended. From the poster's perspective, this may no longer be a legitimate source at all.

I think a comment would be appropriate.

"[E]ncouraging the author to rewrite the post" (from the question) might be a little strong; perhaps something like "X is no longer considered a reliable posek by many; you may want to consider this in your post." with a link to relevant information, and maybe a link to this question.