Michael told Fredo his plan because he probably had a tail on Fredo to see if Fredo would go to Roth with it. It would also serve to scare th hell out of Fredo, and maybe Michael wanted to see Fredo's reaction. In any case, it is clear that after the Michael-Fredo scene, things between Michael and Roth had soured to the point that Michael and Roth were sending each other signals that they were not allies at all. It wa after th encounter with Fredo that michael demanded to know from Roth who gave the order to kill Pentangeli, and Roth's ominous statement that he was going to take a nap, and when he woke up if the monet was on the table he had a partner, but if it was not on the table, he did not.