Building a fixed gear is about more than just style. There are many very good cycling reasons to ride fixed. In the absence of any serious wind or hills, I enjoy riding fixed more than riding geared.
"chopping" a bike is not part of the conversion process any more than removing the brakes. No dremel, file, hacksaw or drill has ever touched any of my frames. And they all have brakes.
There is no more wrong with converting a nice vintage bike to a fixed gear than there is with "upgrading" it with Dura Ace components and high tech wheels. Would you have felt the same if the owner wanted to do that to the bike?
And yet to expect the owner to ride it as a six speed and downtube shifters as just as enslaved by style as is converting it to a fixed gear.