How could a man who has probably witnessed much worse in combat, go to pieces at Louisa's fall?

The first clue is his first coherent words: "Oh God! her father and mother!"." Wasn't it the ship's captain's duty to inform parents when a sailor died? Perhaps his mind leaps back to some moments he's had dispatching this duty. It's almost like a flash back, like we think of with combat veterans. We would call it post traumatic stress disorder, but it's as if he's paralyzed by something for a moment. Something in his mind, makes him reach out for the wall to steady himself. (Mentally, he's still at sea, perhaps?)

JA was an uncanny observer of human nature. This wouldn't be the first time she put her finger on a psychological condition that didn't even yet have a name.

The irony of the situation, that all these competent, fit men are around and they all loose their heads a bit. Just as Sophie must guide the Admiral through, life, I think JA's saying that the most competent and worthy man, occasionally needs a woman's help.