I haven't been through a sudden tragedy like this, but I have extended family who has been, and I've lost a brother and a newborn son to death way too early, so I thought I'd share some thoughts of how you can help a grieving friend.
1. Remember. Mark your calendar now and send the grieving family a card, note, email, facebook message on the anniversary of when their loved one passed away. Just last week was the 2-year anniversary of my brother's passing, and every thought, note, and bit of love that people sent my way and to my parents helped us walk through that day and be honored and blessed that people would remember him.
2. Remember, part 2. Immediately following any death, the family is thrown in a whirlwind of people sending condolences, asking if they can help, making funeral preparations (imagine wedding preparations thrown into five days or so!), talking to family members -- not to mention processing their grief. It's after the funeral where the fact that "life goes on" hits and you have to try to cope with the reality that your loved one isn't there -- this is when family and friends are needed, just as much, if not more than before. Send a bouquet of flowers a few weeks after the funeral just to say you care and are thinking of them. (I remember cherishing the flowers sent after I lost my baby boy. They made me smile because someone remembered.) Write them a note or facebook message any time you pray for them - even 3-6 months from now - and just let them know you care.
3. Pray. So many times when I was grieving, I relied on the fact that people were praying me through this. I couldn't always form prayers because my world had been rocked so hard, so knowing that people were praying for me was such a relief and an encouragement.
4. Bring meals. This is probably most important in the first few weeks and a few weeks after the funeral. If you're a good friend, perhaps you could make them a schedule using something like foodtidings.com. Or you could ask a friend if something has already been set up. It's such a relief not having to worry about dinner when you are trying to process your loss. Even a few weeks after the funeral, you could drop off a dinner or if the grieving family is interested, you could visit for a short while. Anything to help relieve the stress is helpful. Here is a good blog post on Tips for Bringing Meals to a Grieving Friend.
5. Let them grieve. The sadness and tears will come in unexpected ways over the next few weeks, months, and years. Sudden tragedies can especially take a while to heal from because they are unexpected. If the grieving family isn't acting like themselves or is a little snappy or down about life, keep an eye on them so they don't get stuck in depression, but also, give them a lot of grace. They don't even know all that they are processing. Sometimes I have to remind myself that I still am processing the grief of losing the people I've lost in my life.
6. Don't try to have answers. This blog states it well. Please read it - because people, especially Christians, often try to make it better by saying "God's in control" or "it'll be all right" or some other cliche. Those answers do not help. Often they just drag up a ton more questions in the grieving person. It's okay to not know what to say and just be quiet or to be honest and say, "I wish I had the right words, but I don't." Give hugs. Write notes to say you are praying for them and thinking of them. It's okay to not have the answers - we're human, and the best we can do is be there, help where needed, and listen.
There are a lot of other things you can do help a grieving friend, but hopefully this gives you a good start. It's some of what really helped me in what I have been going through. Ask God for His help and wisdom in how you can bless, encourage, and most importantly, support your friend, and He will.
John 14:27, "Peace I leave with you, my peace I give unto you: not as the world giveth, give I unto you. Let not your heart be troubled, neither let it be afraid."