Ok, Believe it or not I do have experience in this. Yes it helps very much. But How he may need to use it may be different than for me. As kava works its own magic. Ok when I first started I drank kava morning noon and at night. Well no one can keep that up for very long. But as long as your fathers Hydrocodone habit isn't too large. Really 7 days, and he should be through the worst.
So my solid plan of attack if I was him, would be to begin to taper his hydrocodone, down to a very small dose. Then clear his schedule for a week.
Take kava by tea, as much as needed. EDIT- Infraredz- (Now Vicodin, Lortab and Norco [and 99% of hydrocodone preparations] are all pills that contain hydrocodone and acetaminophen [paracetamol] NOT aspirin, so he should not use kava, during the taper)
Make sure he eats and takes Fiber. Because the kava could add to the Diarrhea. I can not stress eating and using fiber enough. Or the diarrhea, will not be worth the comforting effects Kava can provide a opiate Withdraweler. ill tell ya, Kava has been getting me through some rough one there. I hate to say it, but I have kicked opiates more times than I care to tell. If ya need a list of other comfort meds, OTC sleep meds, Vitamins, or supplements that will help, please don't be afraid to ask. I am no councilor, or Doctor, but I am someone who has kicked opiates. I'm always willing to help in anyway I can. That's why I'm on the Kava lounge. Need something fun and new, to take my mind off that stuff, I feel I have found that here. With like minded folks. Much love. Roaddog....