Amantadine is actually excreted unmetabolized in the urine. Patients with kidney disease need to have the ok by their kidney doctors before starting on the medication as it can accumulate in the body with impaired kidney function, especially if they require dialysis. There is one study from the Annals of Medicine in 1980 that looked at dialysis patients and levels of amantadine before and after dialysis. THey found that only a small fraction of the drug was removed with each dialysis treatment.
And yes, it can cause hallucinations, especially when it's withdrawn too quickly or if there's too much in the body. On the other hand, dopamine and dopamine like drugs can also cause hallucinations. This can be treated with decreasing the med or adding an antipsychotic medication such as Seroquel or Olanzapine. In your father in law's case, talk to his renal doc to make sure that amantadine is ok. Then discuss your concerns with his neuro. Good luck.