I would not presume to challenge one who is so blind.
The closest you could do is refer to Jesus' statement that the Sabbath was made for man. My response would be that Israelites were men, and this does not mean that man is to be applied in a generic sense.
At any rate, I would respond that Israel had to be taught about the Sabbath, so it wasn't in existence prior to that.
Gen 26:5 has also been used, but all that says is that Abraham obeyed the commands that were applicable to him.
In practice, though, I observe the eighth day because it points to Jesus' resurrection and the believers' new creation in him. The number "8" is associated with a new creation (the first day of the week). Jesus' resurrection was the beginning of the new creation