Well this comes down to what country you are in as well. For now, I'll assume you are in the U.S.
If this is a part-time towered airport, then during the tower's hours of operations, the runway is controlled and pilots need clearance to takeoff/land on the runway. Once the tower is closed, the airport reverts to a non-towered airport. If you told him what you did after the tower had closed, then I think you would be fine (you didn't issue a control instruction) since the takeoff would be at his discretion and his own risk.
This was what occurred for where I went to school at: KBVI. BVI is a Class D airport from 7 AM to 10 PM 7 days a week (closed on federal holidays). If you wanted to take off or land during those hours, you needed to contact the tower. If you were taking off or landing anytime AFTER those hours, then the airport becomes a Class E airport and all operations are identical to any other non-controlled airport (pilots announce their position/intentions through the UNICOM).
It sounds like the tower had just closed and you were still up there when a pilot called in asking for a takeoff clearance. However, if the runway does truly close at sunset, this could be a different story. However, I think the same would still apply (takeoff would be at the pilot's own risk).