Just like the relationship between employer and employee, what is the word for the receiving side of a mention?
An example of a sentence: The mentioner mentions the mentionnee, but mentionnee is not a word...
Another example is if we have two types of people, one is sending mentions they are the mentioners, the other are being mentioned, they are the mentionnees.
This is from I got the term "relational antonym" What's the term describing employer-employee and similar role relations?
This question is predicated upon a false assumption. It wrongly asserts that mentionee is not a word, but this is simply not true.
This is because deriving agent and patient nouns from transitive verbs using the ‑er and ‑ee suffices is productive in English. You can produce them almost at will whenever you need one.
That doesn't mean it is always guaranteed to be possible, but it usually is. Sometimes using an otherwise-productive affix can be blocked by particular grammatical, semantic, or lexical concerns. So for example although you can derive agent nouns like talker from nearly any verb, you cannot derive patient nouns like talkee because talk is intransitive so there can be no patient or beneficiary.
So go ahead and use mentioner and mentionee: after all, everybody else does.
