You never said how many miles were on your brother'* engine.
It'* definately burning oil, just not so bad that you notice the blue color in the smoke.

The reason the smoking decreased after you changed plugs is because the new plugs are clean, and burning so well you get more complete combustion.
If the valve covers & pan gasket have been leaking, that would acount for the majority of the oil loss. It is probably only burning it through the valve guides at start-up only.
I would not be surprised if there is a sludge problem under the valve covers that is preventing proper drainback. That will cause oil to get sucked through the valve guides everytime.

Leave the new plugs in there for now and change the V.C. gaskets.

car just made 100,000 miles

That'* generaly not enough miles on a 3800 to have significant piston ring wear, or valve guide wear. It shouldn't be too badly gunked up either, unless it'* been severely neglected.
You will just have to see what it looks like when you remove the valve covers.

I don't know if these engines have valve stem seals that become brittle, but I have not seen any of these cars that smoke, unless they were severely neglected and beaten to death.