Dog is still OK today. I don't know what else could have killed the snake inside the fenced area. If somebody wants this snake, I have it
in the freezer.
Your Lab probably killed it. They can 'shake' a snake to death very quickly. A Coral snake would have a difficult time finding a place to latch onto a large dog, except for the lips or nose.
They can't simply 'strike' like pit vipers can...because their fangs are 'fixed' and the venom duct is not directly connected to them. Meaning...they need to grab onto an object and chew...in order to deliver any venom.
Just the same, it would be a good idea to watch your Lab. The onset of the neurotoxin venom can take time and 'symptoms' might not be readily detectable.
I had a Chocolate Lab that absolutely hated snakes and would grab and kill any that he found. He was bitten 3 different times by Copperheads, but it never dissuaded him.