Hi Olivesqueeze,
During my wife's four year battle with this disease her hepatologist prescribed 220 mgs of Zinc sulfate in addition to Xifaxan and lactalose to control her ammonia. Until the last few months before she passed, she never had a really bad episode of HE. Her hepatologist told us the Zinc, in addition to the other meds helped to keep the ammonia under control.
Just something you might want to ask your husband's doctor about.
Like Connie said your dad needs to be treated by a Hepatologist and not a GI doc. They have much more training and experience in treating liver disease.