In the absence of a by-law, this is now common property. The Owners Corp can remove the A/C unit and repair common property at their (the OC’s) own expense.
Alternatively, if the lot owner wants it repaired the the OC doesn’t have any fundamental objection, they can allow the lot owner to do so provided they agree to a special resolution by-law giving them ongoing responsibility for its upkeep.
The one thing that should not happen is that the lot owner gets the a/c repaired or replaced at OC expense.