What part Fredo played is not very clear and there is not much consensus on that here. Some people beleive he opened the drapes, some others beleive he did not know that it was a hit attempt.

However, michael did not have to know the exact details of fredo's involvement to say "you broke my heart". Infact he did not know the details when he said that. When he saw Fredo mention the name of Johnny Ola and Roth in the context of going to the Superman show and other such places, michael suspected that Fredo must have colluded with Roth in the assasination attempt.

You may use "search" feature on the top right of the page to find out the old threads discusssing this.