I personally don't think that anything in Snowball was below the belt. But if I were Warren, I probably wouldn't have been happy about the Graham and Astrid stuff. (The whole thing just made me feel really bad for Susan.) I think Schroeder started researching a little before Susan got sick, so it's very possible she got the information from Susan herself. If not, she probably could've gotten the info from Astrid, one of the kids or possibly even a neighbor/friend.
Also, some parts of the book made him seem a little immature. I forgot the instance, but I think there was a scenario where he told people to stop buying a certain company, because he hadn't finished getting all the shares he wanted yet.
eta: I think it made Benjamin Graham look bad as well. Even if Warren knew all about that stuff, I doubt he would want to be the reason for it to be out there.