This man was so valuable, that he was the only person that could freely have almost anything on the screen, excluding hard core porn, instead of IDE, and no-one, not even the board members that came to visit our offices from time to time, dared to say anything.
To give you some perspective, we had this ugly system that was bought for several hundreds thousands of usd for the purpose of processing one specific type of information. The system was delivered by the third party and its implementation into our infrastructure took about a year.
The problem with it was, the time it took to process a single xml message, just the message acknowledgement took 1.5 second in the best case scenario, while the whole message processing took around 30-ish seconds.
It came a moment when it was needed for this system to interface with another system outside our organisation. It had a requirement that all messages should be processed end-to-end in less than 10 seconds. Soft deadline was set for two years and in that period only 25% of institutions (us included) managed to get it done, by using full teams of 10+ developers.
On our side, we had this guy and one product owner. He managed to code the proxy that basically did the whole logic of that old system and just inserted the results in db without knowing the inner workings of existing software in three months.
It is useful to add that when interviewing at the new company, were I came before him, they immediately recognized his worth on the screening interview and he was fast tracked to the employment.