If the councillor declared an interest, they should have left the room. When a councillor leaves the room (even to go to the toilet), they affect the quorate levels as they are not present. The level for quoracy differs if it is full council or a committee. For full council it is 1/5 rounded up and for a committee it is a minimum of 3 councillors.
From what you have written, they should have declared, left the room and therefore could have made the meeting inquorate.