The airlines will NOT reseat other passengers to accommodate an extra place for a lap baby. I really doubt that happened. She might have reseated non-rev employees who were listed for the flight but it's very doubtful they reseated paying customers.
If the airline has open seating, like Southwest, they have better chances. Some parents head to the back of the plane, less likely to fill up, put the baby at the window and then they occupy the aisle seat. Most passengers will avoid a middle seat. Of course, if it's full, the parent can move to the center and last by not least, give up the seat for the lap baby.
Tell them to be really nice at check-in because they're basically asking for something for free.
Never "fly when they nap". I've seen this as an unhelpful "tip". One small delay and you have a big meltdown! Most of us find that late morning is good. Business people are gone. Babies are in better humor in the morning. Delays are less likely and there are other flights later in the day so if anything goes wrong, you'll get there!