Is the answer that contained the phrase "try to reduce it" concerning insulting, reviling, and cursing, a way of making lawful what God has forbidden?

Your statement "try to reduce it" regarding the prohibition of cursing and reviling is not considered an authorization of what Allah has forbidden. This is due to the possibility of your ignorance, or weak comprehension, or the intention of gradualism in da'wah (invitation to Islam) and gentleness, or your fear of the other party's reaction. This phrase, though incorrect, is far from permitting what is forbidden. You should advise him to abandon all cursing and reviling, and repeat your advice and guidance.