The breast will stop lactating and therefore producing milk if it is not being stimulated with feeding. They will naturally then deflate. The skin will not stretch as much if they are not engorged with milk.
You can assure your friend that whether she nurses or not her breasts will shrink back to pre-pregnancy size or smaller, and that therefore she should discuss the other important issues regarding breast feeding with her OB or pediatrician, and scratch the current concern off the list for consideration.
Thanks and best wishes.