I don't know if anyone responded to this yet regarding the main dining rooms. On the Constellation there is only one main dining room. We were on the ship in March and the anytime dining always had a long line when they opened. If they want to go early they would probably wait awhile.
If I were this couple I would definitely go on the first day and see if I could change to fixed dining (either early or late seating). When we were on the ship early was 5:30 and late ws 8:30.