shade can accelerate a rot problem in old wooden sills. However, if the
sills are properly maintained (and properly repaired when needed) the shade
won't make any significant difference.
As many folks have already stated: a hack fix-it job will hide the rot for a
while, but unless he's done (or paid someone to do ) a proper repair job,
NOTHING will stop the rot.

from the sounds of it, he's just a cranky guy looking for something to ****
about (anyone who complains about woodstove smoke blowing across his
property is a nutcase). I'd make happy noises at him whenever he bitches. If
he really bitches a lot, you might get a couple of estimates to have the
tree removed (don't forget the landscaping to repair the hole it will
leave), and mention that you might consider doing it if he foots the bill.
Explain that you really llike the tree, but if it would really make that
much difference to him, you'd be willing to sacrifice the tree on his
nickel. around here, having a largish tree removed from a residential lot,
then having a decent lawn repair done will run in the 3-4 kilo buck range
(especially if you use one of the high-end contractors). I bet that will get
him off the topic really fast......

--JD