Sorry for earlier error. Think of it this way: when you are in the lift and it accelerates upwards, you feel heavier right? This is because the upwards acceleration of the lift causes an additional force to be applied to your feet, increasing your apparent weight. This is why when calculating the force due to weight acting on the lift and the painter, it is appropriate to factor in the total acceleration of the system. The diagram consists of two separate free body diagrams, one for the painter and one for the lift. Though the normal contact force acts on the lift from the bottom of the painter's feet, it is drawn at the top for clarity's sake. Feel free to clarify. There is more than 1 method to do this question and this certainly not the best, but this is one way to look at it.