I know Fredo was dumb but he could have worked out that what happened to Ola was no coincidence. Also, an attempted murder did take place in Roth's hospital room. Anyway, its quite an irrelevant point. I was just pointing out that Fredo did know of some of Micheal's goings on and that he may have tried to use these factors against Micheal in the future. How he would do so is questionable but the possibility still remains, to say that it simply couldn't/wouldn't happen is unreasonable.