It's hard to know what to do when someone close to you dies. People have different reactions and different coping methods that astound some and seem weak or useless to others. There's really no way to know how a person will deal until they have been put into a situation where that is what they must do.
Mal knows this and he knows River, to some extent. She's been on his boat near on five years now and in that time she has developed in ways no other young women he knows of, have. As he expects though, Simon's death proves heavy on her.
As she carefully climbs up the side of the windmill, narrowly being missed by the thick panels of wood spinning around, Mal watches her. He doesn't try to express sorrow for her loss. It's his loss too and he's had enough of people trying to make him deal. He's not about to turn around and try to stop River from doing what she does to cope, crazy as it is.
He just stands there, rooted to the spot, and watches her climb to the highest possible point in the windmill that it's possible to reach without being sliced into many more pieces than a human being should be in. There, tears leaking from her eyes, she stops. Her hair blows about in the wind, occasionally whipping her across the face.
She remains still as ever. She closes her eyes and some amount of panic settles in his stomach. Graceful as River is, there's always a chance that one day she will slip up. She is human.
She doesn't slip up. Rather, she climbs back down the way she came with her eyes still clamped shut. Her moves are precise and controlled. On the ground, she opens her eyes and looks straight at him. Though there are still tears running down her cheeks, there is also a small smile.
He smiles sadly back and gives her a nod.
Her smile remains for a moment longer and then she turns around, hiding it from him if it's still there. Her hair flows out behind her as she moves back to the ship.
Mal sighs loudly into the evening air. People cope with death in very different ways, he knows, but somehow, somehow, it is all the same.