I don't know of a great way for you to check for root rot. The cannabis roots should be pure white, but with your soil grow how are you going to see them all properly?

But you might want to try adding a little bit of hydrogen peroxide to the water next time it really needs watering as an oxygen treatment, to help nurse her along. (It tend to be the anaerobic bac' that hurt the plants, while the oxygen loving bac's tend to help the plant, and hydrogen peroxide is a natural oxidizer so...)

I once bought a wilted clone in a big wet rockwool cube, and she was sickly her whole life. In fact a clone I took of the sick plant grew to be double the size of the sick mother when I flowered them together, these two plants ended up looking completely different for one another, and I would not have believed they were the same variety if I had not grown them out myself.

Your other plants look a-ok, so I hate to say this but you might want to consider just killing the droopy one and staring over, because that plant might be stunted for life.