Carbonated water is fairly acidic and I've gone months without cleaning my soda water kegs. But if he can't chill the cornie, there really isn't any point in using one. He would still have to re-pressurize while the water chilled. I'd just get a couple carbonator caps. If he has two 2L/3L bottles, he can be drinking from one and carbonating the other. He just needs to remember to re-pressurize the working bottle every time he pours so out.
Maybe a #5 bottle would be a better idea, as it would be small enough to sit in most refrigerators. Then he could have a two-way splitter and just snap the connector back on when he puts the bottle back in the fridge.
I have a #5 bottle and get about 50 gallons of soda water per filling.
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