Not so good—Saying nothing at all.
不要说：什么都不说。

This is actually one of the worst things that can happen to a grieving person: having people ignore his pain. If you're not sure what to say, or are uncertain that the person wants to talk about it, it's okay to say just that.
周围的人忽视他的伤痛，这对伤痛的人来说可能是最坏的事情之一了。如果你不知道去说些什么，或者不确定他是否愿意聊这件事，那就直接说出来吧。

Better—I'm not sure what to say but I want you to know I'm here for you。
不如说：我不知道该说些什么，你只要知道我一直都在你身边。