More to the point, the OC is obliged NOT to provide improvements or services that only benefit only one owner unless there is another reason that benefits the whole building.
The key question is, does this fence enhance the amenity, safety and physical integrity of common property.
If not, the most an OC should do is allow the owners of that lot to build the fence with the proviso that they (the unit owner) undertakes to maintain the fence in perpetuity.
This will require a special resolution which will need the support of 75 percent of owners voting at a general meeting.