It's difficult to watch friends struggle with the death of a loved one. You want to express your sympathy and regret, but how? What words can you say or write to bring them comfort? And what if you bring their pain back to the surface? (In reality, the pain is already there, bringing it to the surface doesn't make it hurt any more.) What if you make them feel even worse than they already do? (In reality, that's probably not possible.)
For the most part, just being there for your friend is a good start.