Do you wish to simply stop the diarrhea or are you interested in treating the underlying condition?
If it's just the diarrhea bugging you, give her boiled rice with buttermilk or yogurt or keifer stirred in.
There can be several reasons for diarrhea, all requiring much more information to try to figure out which this hen has. One is egg binding. Is the hen laying? Any egg quality issues recently?
Another is coccidiosis. It usually affects more than one chicken, though.
Another is bacterial infection. Has the hen been digging in decaying compost? Eating spoiled fruit or veggies?