How much rock salt did he throw onto the bed? Try to clean up as much as possible.

Try to scrape some of the surface soil away before the salt percolates further down. Maybe pouring water on it will take the dissolved salt deeper into the ground and so nearer the roots.

However, in the winter I don't think the tree will take up much water because it is dormant, so it could be all right.

Bad argument? Or, "Oops, I grabbed the wrong bag!"?
Either way, the remedy is to scrape up as much visible salt as you can manage, drag the hosepipe over, put a sit-down sprinkler on the end of it, and sprinkle the area long enough to put down at least 15 cm of water--it could take as long as 12 hours. A few watercans full will just make it worse.