If the narcissist has friends or relatives that he has manipulated into thinking that he is a good guy and everyone he knows is a horrible person, then, yes, it can backfire.

My mother was a narcissist. If I would challenge her or stand up to her, she would then tell her friends and all the relatives how horrible I was and that she did nothing to deserve it.

I'll give you a specific example. My parents separated a couple of times but always got back together. The second separation my mother initiated. She was convinced that he was cheating on her so she cleaned out the bank accounts and moved in with my aunt (her sister) and family. I decided that I would try to stay out of the fray which was hard because I lived near them all at the time. If either of them wanted to talk to me, they could call. My father would call far more often than she did. We would visit them both once a week. First we would visit her, then drive to his place. I figured this was fair. She thought otherwise.

Apparently, she badmouthed me to my aunt who was convinced that we were paying less attention to my mother than we were to my father.

Eventually, my parents got back together but my aunt barely spoke to me for several years and treated me badly. This was all because my mother had her thinking that we were spending more time with my father.

I could go on but I just wanted to mention this as an example. If your narcissist has the rest of the family snowed, then, yes, confronting him could backfire.

So...evaluate the situation and then decide how to proceed. If you think he could cause problems for you if you confronted him, then just barely pay any attention to him. Be polite but keep your distance.