Srilankan Airlines — Overbooking

I made a reservation with Sri Lankan Airlines for my cousin to fly from Colombo (CMB) to Toronto (YYZ) on May 01, 2023 with the details as follows:
Ticket purchased: Toronto, Canada
Date & Time of Travel: May 01, 2024, 0325
from Colombo (CMB) Flight No & Route: UL501 – CMB to CDG; Seat 26A UL501 - CDG to FRA; Seat 26A DE2402 – FRA to YYZ with scheduled arrival on 01 May, 2024 at 1800 hrs.
As my cousin was living ~500 km from the airport, he arrived at the airport directly from his home around 6 PM and went to the Sri Lankan Airline counter around 10.00 PM. He was told at the counter that they have overbooking around 10 passengers and he will be put on Qatar Airlines Flight QR 663 leaving 10 minutes later at 0335 from CMB with the following route: QR663 – CMB to DOH QR007 – DOH to LHR AC857 – LHR to YYZ with scheduled arrival on 01 May, 2024 at 1845 hrs. He was treated badly by the Airline staff at the counter and almost threaten and forced to take their offering otherwise he cannot travel until May 03, 2024. Also Sri Lankan airline staff were trying to communicate with him in Sinhala language, which he cannot understand as his mother tongue is tamil with limited English communication skill. Then they tried to convince him in broken English and tamil which he somehow understood and left with no choice other than to accept their offer as he doesn’t want to stay in Colombo at unknown place for another three (3) days with no confirmed accommodation and running around with his heavy luggage. Also we have made plans for him in Toronto on his arrival from May 01, 24. He was booked on Qatar Airlines and issued with the Boarding pass for all three sectors CMB to DOH, DOH to LHR and LHR to YYZ by Qatar Airlines at Colomb Airport.
My question to Sri Lanakan Airlines is why a passenger travelling for more than 24 hrs with confirmed seating arrangement was asked to take another flight on the 11th hr when checking in at the airport?
It was chaotic at the airport and it was a bad and an unpleasant first ever travel experience for our cousin who was getting into a plane and flying overseas for the first time. It gave a bad impression about Sri Lankan Airlines, which is responsible for creating all the mess in this journey. I am expecting an apology and financial compensation from Sri Lankan Airlines for all the troubles and mental agony forced upon us on this journey.
I am hoping to receive a favourable response from the Airlines.
Even though I filed an online complain with my email address in the following link
Feed Back Centre (srilankan.com)
I did not receive even an acknowledgement email from Sri Lankan Airlines so far.

Thank you