This is nonsense. It's based on the flawed pH ideology of quacks. The claim have been that it will change the pH in your intestinal track and thus prevent the viral infection. The truth is your stomach has acid (this breaks down food). When things exit your stomach the acid is neutralized by the pancreatic excretions as they enter the small intestine. The pancreas actually secretes about 8 cups/day of enzymes and bicarbonate. Drinking a couple sips of juice a day will definitely not effectively change the pH in your intestines.
While grapes has been shown to have antiviral properties, it has not been shown to prevent viral infections in humans. According to a 2010 review "research in humans is required to confirm previous findings and to better understand the biomedical applications of grape products in broader populations." Only tests in test tubes have been done. The human body is much more complex.
I would also suggest eating grapes instead of just grape juice. You get more nutrients from a whole grape than just juice which can contain in a single serving (240ml) about 36g of sugar.