It’s unclear what your question is.
If the owner is amenable to paying 50% of the repair, then that’s a win for the OC.
If the owner is not amenable and the owners vote for this 50/50 split, then I’d say the owner will take the matter to NCAT and probably win if as you suggest thst it’s OC responsibility.
Of course there may be an exclusive use by law encompassing not only the usage but the maintenance and repair of that common property. In which case a strata lawyer will need to be consulted for an opinion as to who’s responsible.