It seems evident from his actions and later comments that Wentworth felt responsible for Louisa's fall and was struck dumb with horror. I think it is evident that her cares about Louisa and especially about her family ("Oh God! her father and mother!")

Mary starts screaming "She's dead! She's dead," and Henrietta, still standing on the steps, faints as a consequence. Benwick and Ann have to catch her and keep her from falling too. Meanwhile, Mary is clinging to Charles like a limpet, preventing him from helping.

I also get the sense that all this is happening in real time - in the time it takes to read the scene. Seconds seem like hours, but it all really happens very quickly.

I agree with everyone who makes the point that Wentworth and Benwick were experienced officers, and yet they were still at a loss at first. They are out of their element, especially with women. Ann is the only one who keeps her head. That must have made an enormous impression on Wentworth. It completely changes his view of her qualities: she's not a flake, she's deferential; she's certainly not weak, she's someone you can count on in an emergency.

She is the only lady of their party who is able to make a contribution, to make things better. Louisa causes the accident, while Henrietta and Mary impede the rescuers.

Ann is able to direct personnel,("Go to him, go to him," cried Anne, "for heaven's sake go to him. I can support her myself. Leave me, and go to him.), initiate treatment ("Rub her hands, rub her temples;") allocate resources ("here are salts: take them, take them")and when that doesn't work, send the right person to seek medical attention ("A surgeon! Captain Benwick, would not it be better for Captain Benwick? He knows where a surgeon is to be found.") Everyone else is wrapped up in their own misery and horror and is barely functioning. Ann not only attends to her task with Henrietta, she looks out for everyone else too.

Wentworth, who is used to acting in a crisis, can't help at later reflection to recognize her clear and shining superiority to everyone else (himself included) around her.

Up until now, whether you think he was cruel to Ann or not, his actions clearly indicate that he doesn't respect her. This is the moment where everything changes. This is the point where she wins his respect which she had lost when she ended their engagement.