He did something wrong that wasn't the hot water. Did he dry and oil right away?
If two guys use hot water and one gets rust and the other doesn't. Where is the problem?
Absolutely - Peter would not even crack a beer or have a drink of water after shooting before cleaned his rifle that Taylor built him.
He used boiling hot water - every time for cleaning and. He immediately ran an oil patch down the bore after flushing the barrel and it always came out red- rust!
The effects of this flash rusting was accumulative on that barrel to the point it had fine pitting, one end to the other, very even.
I think perhaps 12L14 flash-rusts more quickly than does the harder steels used by GM, Colerain and perhaps some others.
I use cold tap water as one of the most famous of all gun manufacturers says "USE NOTHING BUT COLD WATER". Do no use hot water due
to the flash rusting and certainly NEVER use boiling or hot water in valuable antiques. That company is Holland and Holland, makers of fine
rifles and shotguns from the very early 1800's. I am not talking about the American company who uses that name - perhaps they spell Holland with
only 1 "L". The SXS rifle H*&H built for Prince Charles a number of years back cost him 250,000 Crowns/Quid/Pounds. They have an idea of what
they are talking about & told my friend he will ruin his valuable black powder SXS rifles and shotguns if he used hot water in them.
After reading that letter, back in 1977, I quite using hot water. The barrels of all of my rifles and smoothbore shine like brand new. Hot water
is not necessary & indeed, does not clean any better in the first place. In the second place, it is another step not needed. In the third, you take the
chance of ruining your barrel is you do not possess one of those remarkable barrels that we are told does not flash rust.
Like Taylor said, ANY rust is like cancer. It's effects are accumulative.