Well, white grape juice may be an OG of around 1.045 or so. 1 pound of sugar would add 46 SG points per pound. A gallon of water would be 1.000.
So, it looks like the OG would be around 1.200ish. It really depends on how much natural sugar the juice had.
Most wine yeasts will die out at between 12-15% ABV. A yeast like champagne yeast would go up to about 18% if it wasn't too stressed.
The result would be a very hot, very sweet 18% "wine". If he used a lesser attenuative yeast, it might have died out before it got to 18%, though...
With half that amount of sugar, it'd be about the same ABV but not as sweet. When the yeast dies off, the alcohol can't get any higher.
We're about making good tasting beverages, though, not hooch. If you want to make a nice tasting and easy wine, that's fine. But any hooch threads will be deleted.
Broken Leg Brewery
Giving beer a leg to stand on since 2006