It's beginning to turn a little purple here because of the cool nights. That and increasingly shorter daylight hours are slowing all turf and most summer weeds down. If a weed isn't growing, it will not die. Communicate to the customer that spraying it at this time would do more harm than good. If it is large patches I would be on it as soon as it shows itself again next spring. I spray buttonweed in St. Aug with 1oz/acre metsulfuron+MSO. I have a friend who spikes this with a herbicide called Spotlight. He swears it actually will eliminate Virginia Buttonweed. I have only been able to control/supress it with the straight metsulfuron/MSO mix I use.
As far as I know, there is no pre-emergent to control it. If you find that there is one, please let me know.