I do not actually do pressure washing vocationally but have researched it pretty extensively as a possibility. I have learned a ton of information here and have put a lot of the information to good use doing some washing with the GC I currently work for. I am now a licensed electrician and am in the process of going back to electrical work as my full time gig.
I definitely agree with the general consensus above about taking the hit and paying for the GFCI. That being said, $400 to change out a GFCI in most markets is gouging at best. An exterior GFCI does not warrant an emergency call (which I define as anything outside of normal business hours or paying a fee to jump to the front of the schedule).
Corbin, in your situation, it may be too late, but moving forward, I would specify that I would not pay for any repairs that I was not contacted about prior to the homeowner authorizing the repair. In our area, you could have gotten that done for $150 to $200 by any number of licensed reputable companies. Again, I realize that geography could make a big difference but even at that, it sounds high to me.
As service professionals, I think we can help set a standard of expectations among our customers. One of those expectations should be that if the contractor breaks something, he is contacted and made a part of the quote and repair process. Just because a customer is okay with being gouged for a repair, it doesn’t mean we should be okay with it being passed along to us.