I really like Erica's suggestion.
Having something traumatic like this happen is never expected, and your reaction is completely instinctive. Just like having your first baby, you don't know how you're going to react until you're in the situation. At least, that was my experience with a cancer diagnosis right after the birth of my second child.
Do the unexpected. Everyone will make meals, put photo albums together because that's what most people feel would make them better. Sometimes, it's the simple things like just being there. A stranger, especially one who's gone through a difficult situation (such as a sick child), can offer support in a unique way.
Google is a wonderful tool. If you can get information on him, where he attended high school, college, hobbies he had, philanthropic organizations he was involved in, etc. use those as your inspiration.
Depending on how he lost his life, recognize him in a special way. The nicest thing anyone could do for me during chemo and after was to wear a LIVESTRONG bracelet in my honor. When you noticed someone had taken them off, it hurt. Purchase her a special token to wear so she always has a piece of him with her, and wear one yourself in his honor.
At the end of the day, life is about the people we get to spend it with. Time does take the agony away, and some people instinctively deal with it better than others. But, honoring the time she got to spend with him and the contributions he made in this world, you'll be honoring her as a parent as well. Pperhaps finding a book authored by another parent who's recently lost a son - I heard a mother on Dennis Miller's radio program whose son was the first Navy Seal killed in Iraq and had written a book about how proud she was of the person he became despite his untimely death.