"I could be wrong, but I told him it didn't stand a chance if he brought it inside ..."

Not at all, you're absolutely right!

Resin

A 6' Doug fir with a B&B rootball is a BIG, heavy thing to move about. Is he up for that? Cuz a lot of moving is involved. First you need to acclimatize the tree - generally they suggest storing in an unheated but above freezing location (like a garage or covered porch) for 10-14 days. Make sure rootball is thoroughly watered before bringing inside (gonna increase the weight). Keep in the coolest possible location, well away from any heat vents and fireplace. Leave in the house NO LONGER than 7 days - 4-5 is preferred. If decorated, turn off lights as often as possible and if you can manage it without wrecking the ornaments, mist daily. Make sure the rootball never dries out - some growers recommend dumping a tray of ice cubes on top of the rootball daily. As they melt, they will soak into the burlap covering and keep the soil hydrated. Once the tree is ready to go back outdoors, you will need to reverse the acclimatization process before planting.

The biggest mistake most folks make is attempting to enjoy their living Christmas tree indoors for too long. These plants cannot stand the heat and and low humidity for more than just a few days without drying out or breaking dormancy, both of which will compromise their future substantially. And DO NOT use any preservative or nutrient solution in your watering regime.

Better idea: plant the tree and decorate it outside! He can use scads of lights, and bird friendly decorations (fruit, popcorn, suet, etc.).

Then he can support the multi million dollar tree farm business and buy a precut tree for inside.

i did it with smaller ones ....

i closed the heat off .. so the room stayed 65'ish .. closed doors to the heated areas .....

misted it in the mornings .. so it had all day to dry before the lights were on at night... never left the lights on for very long ....

spread plastic on the carpet .... to protect that ...

stuck the B&B into pots for easier moving ....

got it outside ASAP after Xmas .... into predug hole ... threw it in .... ball and all ... and mulched extremely well ... dug up and planted properly in spring.... because in zone 5 ... the ground might be frozen after Xmas .... might not be the case in z7 ...

i like rhizo's idea ... set it up outside in front of the picture window ..... and then plant it afterwards ....

really now ... do you prefer to be the know-it-all brother... or would you rather let him have his fun .. and bite your tongue .... in other words... is it better to be right .. or have family harmony with his sister ..... YOUR WIFE ..... or i suppose your sister .. anyway ....

my best advice.. let him screw it up .. and have a good laugh with him afterwards .... whether he succeeds or not ....

ken