If it's a fresh water tank, it's fine. In fact, it will be good.

When I had aquariums I used to use the water for my houseplants. I only used what they needed. They thrived.
Changing 1/4 of the water, 42.5 gal. weekly, seems excessive. He may have a chlorine issue.

Well he doesnt have anything in it right now...its empty sitting on my living room floor (sarcastic yeah!) but thats what was suggested to him. glad to hear we can put that water to good use though!

Quarter water changes weekly is the general recommendation. Not at all excessive.

Freshwater aquarium water is great for plants, depending on what chemicals he uses in it. If he's raising cichlids or something that wants really alkaline water that may not be good. Any time the water is medicated would not be good. Small amounts of salt are ok. I add 1/2 teaspoon per gallon of salt to my goldfish tank, as few fish want totally salt-free water, and I still dump the water changes on the rhodies out front.

Fish poop is fertilizer after all.