Assuming there wasn't sufficient space for this WL'ed ticket to get confirmed, the traveler had applied for allocation of special quota. It was seen that confirmed berth is available in the same train, but in 2AC Class. Henceforth, after obtaining the consent of the traveler, he was provided berth in 2AC Class against the same train.
Usually such instances takes place after obtaining the prior permission of the traveler (Once the traveler has applied for special quota). As a result, the traveler is not entitled for difference in the fare as refund for traveling in the lower class.