That is super awesome. If you decide to sell, make sure to offer it to him since consecutive serials, despite mismatch, are very rare in any case.
Secondly, most of the late 1939 pistols are mismatched. The stories I've been told are either it is the hurry due to the Germans coming in, as well as the Germans putting together whatever was at hand.
Finally regarding value, these pistols are nowadays extremely sought after in Poland. They pay crazy prices for those, what has raised the value of them all over the world. A barrel mismatch 1939 pistol (with a barrel that appears to have a crack on bottom too) that got listed on the "German Gunbroker" right now passed € 7000, and this still three weeks prior to the auction end. It isn't my auction, but since it is still live I don't want to post a link. Again, different continent and different finish as well as "less mismatch", but still interesting to compare against with.