What is the ruling regarding this friend who insults the Messenger (peace be upon him)? Is the questioner considered ignorant? And did he cause the Messenger (peace be upon him) to be insulted and reviled?

Insulting the Messenger of Allah, peace and blessings be upon him, is an act of disbelief that expels one from the fold of Islam. It is incumbent upon anyone who insults the Messenger to repent and seek forgiveness. If you know that your discussion with him will lead to him insulting the Prophet, then you must refrain from it as a preventative measure, in accordance with the Almighty's saying: "And do not insult those they invoke other than Allah, lest they insult Allah in enmity without knowledge." You should also be wise in your advice, avoiding anything that might provoke the one being advised.