How to identify the responsibility of the security guard of the property company for hurting the owner during the decoration? Seek an answer

Cause: Six months ago, because the materials purchased by the decoration company were sent to the front of the unit building, the workers moved them home. At this time, when the security guard of the residential property company passed by for inspection, the temporary gypsum board fell and hurt his foot. The owner knew nothing about the above situation and was claimed by the security guard. Recently, he sued the owner to the court and demanded compensation for a series of expenses such as medical expenses. How to determine this responsibility? Thanks for consulting again. The owner was not on the scene at the time of the incident, and the security and property management company never contacted the owner to discuss the matter for more than half a year afterwards.