Is it worth it to argue with this guy?

I know this guy who is constantly trying to debunk the Holocaust. I realize that engaging idiots on such a level is fairly useless most of the time. Still, I can’t help but feel like it’s such a shame to just sit by and allow someone to spew ignorance and hatred all over the memory of innocent victims. So:

a) If you think trying to address the issue is a good idea – what would be some good resources for me to look into? Obviously the most useful tools would be websites, books, etc. that present verifiable statistics.

b) If you think trying to address the issue is a horrible idea – what would be some alternative approaches – if any?

Thanks!