Grandson still sucks his thumb when he's sleepy at our house. He is 6

Only at our place, and only at bedtime or in front of the TV. His parents say he doesn't do it at home. As a doctor I know the textbook answer. As a dadu I find I don't want to make a fuss and spoil the one evening a week he stays with us. Did yours do this at this age, and did it stop on its own or did someone have to step in?