Pipe seized to inaccessible coupling - how to attach a new pipe?
As stated in comment, it's really very unlikely that the steam supply is high pressure steam (per the standard nomenclature of the industry) for a multitude of reasons - too hot, requiring a dedicated, licensed boiler operator, and the commissioning and annual inspections would have failed the copper pipes before it was ever fired up. Also, someone would probably be dead following a burst pipe (which is why there are all those restrictions on it.)

The short answer for your friend with the down machine and a seized fitting is to call a real plumber or steamfitter and get his operation back on-line, if (we presume) this is costing him money by not running and he's trying to save money by fixing it himself. Down-time is expensive (since the machine is not making money) and the expertise of a professional can cut downtime, making it well worth the brief pain of paying for professional help.