I have two cents to add to this. In a situation like this, I wonder if it might be of any benefit for the owner of the stolen bicycle to post on local craigslist or facebook a phony ad for "vintage bicycles wanted for sale" or something like that. Maybe have a friend do it so that if the thief finds the ad and inquires, that location of the ad poster won't be seen and tip the thief off. Perhaps that would be something to consider doing. And of course always check craigslist/facebook marketplace locally for the bike coming up for sale. Check pawn shops in the area, inform police, local flea markets, the more obvious options to check.
And one other thing, I think we all can use this situation as a learning experience. If a person owns a bicycle as lucrative and valuable as this, perhaps they might should consider installing something where they store the bicycle, in which the bicycle can be locked to it. Such as a bolted in steel pipe, or large wooden beam. Especially if the storage location is a less secure one such as a small shed or garage.
It may also be a good idea to consider a GPS tracking device or air tag, which are fairly inexpensive from what I have heard. Especially on a bicycle like this where there would be ample places to hide one. This is a situation where retrospect is 20/20 vision, but it can serve as a lesson for everyone. I sure do hope and pray and wish him the best of luck that he finds his bicycle again, and hopefully unharmed.