The POH does not mention any limitation on weight for the seat.
The CFI could be right as far as some-one in CASA telling him that. As the next step I suggest that you have a chat with the CFI to find out more - specifically to get the reference source for that statement. i.e. what approved data is there that determines this limit that he refers to?
(No need to use the loading graph as the loading form has the moment arms.)
PS - always useful to get statements like that in writing - from CASA or whoever. It seems to focus people's mind when they get to sign their name to something.