Way to start a holy war

So, last Friday I realized my Google calendar (I have added Muslim holidays) read that it was the "Sunni celebration of Muhammad's Birthday".

Honestly, I'm slightly less ignorant about Islam than I used to be, but still pretty naive.  I was trying to figure out whether Nasreen was Sunni or Shi'a. I know there is a difference, and that they fight a lot.  Something I read said the disagreement started over who would be the next leader after Muhammad died.... but did this holiday apply to Nasreen, or not?

"Nasreen, are most Muslims in Pakistan Sunni or Shi'a?"  I figured this was a safe way to ask, "Are YOU Sunni or Shi'a?" without sounding quite as ignorant.

Nasreen replied, "Shi'a are not true Muslims."

Ouch. 

I can see how a few arguments could get started when relatively easy going 16 year-olds make statements like that.  Imagine what an opinionated group of adult men could conjure up!