"It has a fair bit, and in many households the dryer is run at least
every other day, for
around 45 minutes at a time. "
If it's run every other day for a load of wash, that's not very much
water at all.
"The furnace-mounted humidifier will TURN OFF when humidity levels are
where you want them
to be - that's what a humidistat is for. A dryer re-direct vent will
moisture into a small area of the house, regardless of how high the
humidity might be. "
Yeah, we know that. But the amount of water from a dryer every few
days isn't going to amount to much in a typical home in winter. My
furnace mount one runs frequently during the winter. The diverter
gizmo he's talking about can be switched. If it starts to get too
humid, all you have to do is set it to go outside instead. I don't see
the humidity thing being a real problem. He could certainly try it and
see what happens.