I've seen white Windows sprayed grey,but they've also sprayed over the white silicone around the window frames,after a short time the paint starts coming off of the silicone,what surprises me is I thought it was common knowledge that you can't paint over silicone.
At the end of the day the end result will depend on how good the preparation is and how good the sprayers was.
Did he want to do a good job or spray and dash ?