Bathroom leakage from upper floor to downstair flat : who should pay expenses for repair?

Hello, My dad is owning a flat ( 4th Floor flat) in a society and that flat is rented out to a tenant. Flat is on 4th floor and 3rd floor owner is saying that they are facing water leakage issues and mostly those from bathroom from 4th floor flat and they haven't done any structural changes in flat since we bought it in 2013. But Lower flat owner has done some furniture work till celling exact same location where leakage is showing issue This problem of leakage is started around 10 Mar 2020 and there was conversation on WhatsApp going on between owner of 2 flats on how to bear the cost of repair. Around 20th Mar, corona started in india so lockdown started soon and it was difficult to do any work on flat for repair since everyone was under lockdown situation. In between time, as a fix, 4th floor tenant stop using the bathroom which was causing main issue and he used only one bathroom. That solved problem temporary. As leakage problem got started again, lower flat owner started to ask upper flat to fix the issue. Our stand since we are owner of 4th floor flat ( upper floor flat) was that we are trying to help lower floor flat. But we cannot take all ownership for all expenses since 4th floor flat has done no structural changes in flat at all. Expenses can be shared by both parties equally. Or we suggested, once fix leakage as per standard producers of using fixit solution and cement feeling in tiles. All discussion was going on whatspp chat and in between lower flat owner called and agreed that he is ready to share expenses but fix should be like changing the tiles completely for both bathrooms. Then it is decided that both party will share the expenses so no need to fix leakage by cement solution. Later next day , suddenly, lower flat owner makes complaint to society that upper flat (4th floor) owner is not at all cooperating and therefore society secretary sent a WhatsApp message that " Since two months society is trying to ask you to fix the issue and if you don't do it immediately, society will stop flush water connection for 4th floor flat" 1. Now because of corona, society was not allowing anyone till 3rd may 2020. After that they allowed to come to repair person so it is going to take time because of corona lockdown. 2. Society just got involved before 4/5 days like 5th May in all this matter, so how come secretary is saying that they have sent me notice 2 months back? 3. Without listening complete side of 4th floor owner, how society can order that problem must be fixed by 4th floor flat in next 3 days else they will stop garbage collection, water supply despite the fact that 4th floor owner has paid on maintenances on time . Society is saying that we cannot ask builder for this but owner for 4th floor should bear all cost to repair it. Why society is taking decision who should bear cost. Does society has writes to stop water supply if there is a dispute between two flat owners on expenses c