actually, what I think the best way to do this from Bob and Corrine's perspective is NOT tell anyone about the fake idol directly- and only talk to one another but be deliberately overheard, most likely by Ken or Crystal. let the fivesome scramble within themselves to vote out one of their own for self protection since they believe Corrine is playing an idol and anyone of them is going home.

If paranoia works, when pressed or cornered (as the plan should be) Corrine hints that the idol is from the beach. Self protection spreads like wildfire -Sugar and Susie already suspects they'll be a target of Corrine's and they too will need to cover their butt. It's a gamble, but Corrine and Bob would most likely get the Fang five to have a target within themselves or at least split the vote enough that there is a 2/2 tie between Corrine and another and then the third person with three votes goes home.