You might want to compare your friends Hopf with the two I found for sale in a quick search

Here
and

Here too

Maybe one of these looks like your friend's?

Given the number apparently made with the Hopf stamp on them, it is unlikely that the history of a particular instrument can be found by working down from the many makers. A better bet would be to try to track previous owners, it there is any info available.

You may have already found the site of another owner HERE. I didn't note whether he gave contact information, but maybe he would have more info?

He looks friendly enough????

John