Your Executive Committee (or strata managers if they have delegated powers) very quickly need to issue a Notice To Comply citing relevant bylaws about changes to common property and unauthorised alterations and renovations.
My understanding is if unauthorised changes are made to common property and the unit is sold on, the Owners Corporation inherits responsibility for the common property as it was at the time the unit was sold.
The Owners Corp won’t be obliged to return the unit to its previous state but they will be responsible if, for instance, what could be very dodgy plumbing causes a problem. So you need to get a paper trail in place as quickly as possible so the owner has to accept responsibility for the unauthorised work before they sell the place and ride off into the sunset and the new owner passes the buck back to the Owners Corp.