Runny/watery poop is usually caused by either the bird being too hot which causes them to eat less/drink more OR by the bird just not eating enough due to some other reason.
How hot is it where you are (in the coop)? Do your hens have enough ventilation/water to keep cool?
You said this is the only hen with this problem - are the other hens the same breed as her?
How hot is it where you are (in the coop)? Do your hens have enough ventilation/water to keep cool?
You said this is the only hen with this problem - are the other hens the same breed as her?