With a little help from a friend and the dark of night, i now have winshield washer fluid in my brake fluid on my F-250. I suspect about one-fifth of a gallon.
What is the next step? does it need to be completely drained and filled with fresh fluid?

 

You could try to remove as much of the mix as possible first, then, yes, a thorough purge of the entire system is the best suggestion I could offer.

 

Have you pressed the brake pedal since you discovered this? If not, you might be able to get away with suctioning/vacuuming out all the fluid and replacing it with brake fluid. If you pressed the brakes even once, I would do a complete purge.