He’s a friend, right? And you’re only a reader, not a literary critic whose credibility will suffer if you don’t point out every flaw. Find SOMETHING you liked about the book and mention that on the Amazon page. You could call it a “page turner” because you had to turn from Page 1 to Page 2, didn’t you?
In a case like this, less is more. And remember, you’re doing this in the capacity of being a friend, not an English teacher.
If the funeral has been held, your friend does not need flowers. At a time like this, she could use some moral support. Call her and tell her you would like to bring some food over and ask what she may need. Then bring it to her, and be ready to listen if she wants to talk. Your presence and your caring will be meaningful, because when a tragedy like this happens, people don’t know what to say, which isolates the person who is grieving.