Well, you can probably eat the potatoes if they're ok, but don't leave any in the ground overwinter. And, as I said, I don't know if you need to dig that plants right away. All my experience with late blight has been in tomatoes. Once it started in tomatoes, you either treated heavily with Daconil fungicide or you kissed your tomatoes goodby. Serenade is a new product on the market that is organic and is supposed to slow late blight down long enough for you to harvest a crop, but I have not tried it.
There have been a lot of articles about late blight this year, though, and they all say that it only overwinters in live tissue, and that most likely culprit in northern gardens is potatoes left in the ground.
So, maybe you can treat now so you can harvest your potatoes, and check carefully to make sure you get all the tubers up.