Can the lot owner identify the building defect that he claims has caused damage? If not, and since the OC has a plausible alternative explanation, it sounds like you have a reasonable case. You could offer the owner a chance to make the repairs and say the OC will make the repairs and bill the lot owner if they are not made by a certain time. Take lots of photos, have an expert report (say, from a structural engineer) showing that there is no defect which would have caused the damage and suggesting that leaving windows open is a more likely cause of the damage. You might need the photos, expert opinion and paper trail if you go to the tribunal to recoup the OC’s costs for the repairs.