That is the most difficult question there is.  If you are grappling with that question, just imagine how hard it is for your friend.  Everyone wants to know "why" when something bad happens.  The truth of the matter is, only God knows why.  He has a purpose and a plan for everything.  I know it doesn't make sense to us right now, but God will bring good out of this.  Now, I don't suggest you tell your friend that.  Right now your friend won't be able to see any way that good could come from such a terrible loss.

I personally believe that nothing happens by chance.  I don't have all the "whys" figured out about my son's death, but that's alright.  I am learning to accept God's will for my life and for my son's.   You might ask, "How could God allow this to happen?"  Let me remind you of who God is.  You see, God is God.  Who are we to question His authority?   That doesn't mean we're weird for wanting a reason for going through so much pain.   That is certainly normal. 

Your friend may ask you "why".   My best advice is to say, "I don't know."  And give them a big hug.