I have not used a flash outdoors with the D415 myself. I did recently conduct a darkness test with the D415 though where I darkened the light in a room by closing the blinds and curtains and turning off the lights and then switched all the lights on at the same time. The camera adapted very well to the sudden change.
I had the default 'Enable auto-exposire' setting enabled in the RealSense Viewer during the test, so the good results in this test would indicate that you should leave auto-exposure enabled too and let the D4 Vision Processor chip inside the camera handle the processing of the sudden light change.
According to a RealSense technology seminar that Intel gave on Tuesday, the 400 Series cameras handle outdoor conditions much better than previous generations of RealSense camera.
I remember that there was a user who smoothed out their lighting by putting a photographers white LED wall behind what they were scanning.
If you are scanning a moving object though, this would not be a practical solution. In that case, I would see if the camera's auto-exposure feature could cope with the lighting changes in real-time.