Exposed Cards Ruling

[keilcase]

Can someone give me a correct ruling on the situation below that happened on Tuesday at our multi-table home game...

Middle stages of the tourney, a player in late position goes all in after the turn. It's folded around to a person two spot left of the BB. He begins counting his chips to decide if he wants to call the all in. The All-In player then flips over his cards because he thinks the person has called.

What is the ruling in this situation. Are the cards mucked and the pot split? Does the person still have the option to call? Or does the person with the option to call win the hand because this could also be a strategy play to get the person to fold?

Any help would be appreciated...