I think what you saw with your rhubarb plant is fairly typical of a rhubarb plant in summer, especially if it is a young plant and you are not yet harvesting the petioles. Rhubarb is typically harvested in the spring, before the heat of summer sets in, and will start to enter dormancy when conditions are hot and dry. Most people don’t notice as they have been busy munching on rhubarb pie! If you are still nurturing your young plant however, the leaves will start to brown and will eventually just drop off. A dormant plant does not need as much watering and it is likely that a good soak would have made little to no difference to the leaves. Fear not, your rhubarb should start growing again when the weather breaks and you should aim to grow it for 3-4 years before you begin to harvest any petioles from it!