Answer: Washing the chemical off immediately and thus diluting it is usually the most important thing with a spray application. I would suggest you contact the manufacturer of the crabgrass killer and ask them what else if anything you can do.
In the meantime, to help the hostas recover, keep the soil evenly moist and remove any damaged foliage. Watering at night may attract slugs so you might discontinue that. The plants may be able to send out new leaves from the crown.
I'm sorry about your plants.
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