The picture is all he feels about it, all he thinks worth preserving of it, all he invests it with. If all the qualities which a painter took from the model for his picture were really taken, no person could be painted twice.
Since the model he so faithfully copies is not going to be hung up next to the picture... it is of no interest whether it is an accurate copy of the model.
I am only interested in painting the actual person, in doing a painting of them, not in using them to some ulterior end of art. For me, to use someone doing something not native to them would be wrong.
And, since the model he faithfully copies is not going to be hung up next to the picture, since the picture is going to be there on its own, it is of no interest whether it is an accurate copy of the model.