Fear not. You don’t have to put up,with this.
But your SM should be more helpful. Sounds like they are one of the lesser quality SMs out there.
Firstly determine if the adjacent bathroom is an original one. If that is the case then the OC is responsible for repairing the waterproofing. The OC has an absolute responsibility to maintain the common property and can effect repairs despite the protestations of the owner.
Secondly if the bathroom has been renovated by way of new wall and floor tiles then check for a bylaw stating that responsibility for waterproofing is on the lot owner. If you find such a bylaw then ask the SM or your committee to issue a bylaw breach notice to the lot owner. He has to repair it or get a fine and can be directed by the court to effect a repair.
If a by law does not exist it is still up to the lot owner to fix the problem.
If the owner still does not want to cooperate, ask the SM or the committee to effect the repairs and to pass the cost onto the owner.
You don’t have to suffer because of the intransigence of the lot owner.
If the SM and the committee won’t help, just apply to NCAT to get an order to get the repairs done. The court can sort out whether it’s the lot owner or the OC who pays, but in any case you get your repairs