Haloperidol is complete, utter shit if it's not prescribed to you. It is no catalyst of any sort, as others told you it will increase sedation, but not in a good way and it would be a shameful waste to combine it with diazepam recreationally. Haloperidol is a dirty drug with dirty side effects.
It's prescribed for schizophrenia, acute psychotic states, and delirium... that might explain why the OP's friend is making that shit up?