He really didn't do anything wrong.
Is washing your LR with freshwater the best idea? No. However, this "washed" rock was never put in a fully cycled tank. It was placed in a tank that was then cycled. IMO, no harm was done. If it had been placed in a tank that was done cycling, that would be a totally different story.
IMO, it's not the best method to get rid of unwanted hitchhikers, but it is effective none the less.
I have found the most effective methods to be:
1) place rock in a bucket of water with an extrememly high salinity. This usually causes the hitchhikers to come out. However, this method can also kill things and create dieoff, just less than using fresh water. But no method is without its faults.
2) Place rock on a ledge with a bucket under it. If the rock is out in the air for a few minutes, most hitchhikers, specifically the bad ones will usually crawl out of the rocks in search of water again. Again, 10 minutes isn't really going to harm your rock, but it too can create a tiny bit of die off.