Proof/confirmations of a TF multiverse needed

I need help with something. See, I know this guy who insists on believing that all Transformers series occur in the same single universe. One reality. One history. One timeline.
He believes that things like the Unicron Trilogy, the movies, and Animated all occur sometime in the future after the G1 cartoon and before the Beast Era point in time (300 years post-G1).
I know well better that this is not the case and that there are a vast numerous universes in TF media, but he thinks otherwise.
I would like to know where one can find proof or confirmations that there exists a multiverse in TF media and not one single universe.
Does anyone know where such statements can be found? Either within any TF fiction itself or from the mouths of any of the creators, any indications would be most helpful.
He believes that things like the Unicron Trilogy, the movies, and Animated all occur sometime in the future after the G1 cartoon and before the Beast Era point in time (300 years post-G1).
I know well better that this is not the case and that there are a vast numerous universes in TF media, but he thinks otherwise.
I would like to know where one can find proof or confirmations that there exists a multiverse in TF media and not one single universe.
Does anyone know where such statements can be found? Either within any TF fiction itself or from the mouths of any of the creators, any indications would be most helpful.