If someone has acute gastroenteritis and drinks prune juice. would that get rid of the virus faster?

Unlikely : The process of acquiring a gastroenteritis involves the virus invading & reproducing in tissue in your body.Much of that invasion involves the GI tract, which looses some of it's ability to work normally until the body eliminates the virus.Keeping well hydrated is important to recovery, but many juices, specifically prune juice, will actually promote diarrhea.I'd stick with re-hydrating solutions.

NO: It would not have any effect.