I'm a little new to the vintage side of cycling so maybe someone will correct me...

But if it had been ridden a lot, a new chain is not uncommon. Sometimes if a very warn out chain is ridden for too long it can damage other components, like cogs and chainrings. Upgrading from a 5 to a 6 speed would not be out of the question. Perhaps at one time your neighbor was quite in to cycling and put on LOTS of miles and wore out the stock freewheel, got enticed to upgrade when it came time to get a new freewheel and went for the "full meal deal" and got upgraded shifting gear too. That's assuming you can go from a 5 to 6 speed freewheel w/o a hub upgrade.

That's one hypothesis.