First, passengers who want a full refund are only eligible if they cancel their flight within 24 hours of booking or before the flight departs.
If it has been more than 24 hours, he can still cancel the flight but will be required to pay the cancellation fee.
Brussels Airlines Cancellation Policy says that Passengers may cancel their flight up to 4 hours before flight departure, subject to cancellation fees and only refundable airport taxes and other government taxes.
https://www.cancellation-policy.com/brussels-airlines-cancellation-policy