Fellow Houstonian here & your local ace will have liquid microlife fertilizers, it's local, it's amazing. Spray daily for a week. Digging & replacing is last resort

There is only one type of spray herbicide that works on St Augustine and does not kill the St Augustine. Those contain atrazine. If the bottle your husband used did not have purple on the label, it was probably the wrong stuff.

Always mow at your mower's highest setting. St Augustine can go an extra week or so without mowing with no harm. It will spread to fill in, better and keep weeds out better, when mowed high.

Watering: Deep and infrequent is the mantra for watering. This is for all turf grass all over the place. Deep means 1 inch all at one time. Put some cat food or tuna cans around the yard, and time how long it takes your sprinkler(s) to fill all the cans. Memorize that time. That will be the time you water from now on. My hose, sprinkler and water pressure takes 8 full hours to fill the cans. Your time will likely be less. I like gentle watering. As for watering frequency, that depends on the daytime air temperature. With temps in the 90s, deep water once per week. With temps in the 80s, deep water once every 2 weeks. With temps in the 70s, deep water once every 3 weeks. With temps below 70, deep water once a month. Note that you have to keep up with quickly changing temps in the spring and fall. This deep and infrequent schedule works in Phoenix and in Vermont, so it should work for you. The reason for deep and infrequent is to grow deeper, more drought resistant roots and to allow the soil to dry completely at the surface for several days before watering again. If it rains, reset your calendar to account for the rainfall.