Thanks for the replies. I agree that this seems innocuous as all betting has finished. However, as a matter of etiquette, I don't think it is appropriate. Compare the wording of rule 59 to 60. First, rule 59 says that players are obligated to protect other players ... at all times. While rule 60 says a player shouldn't expose his cards with action pending. Obviously, criticism shouldn't be tolerated at any time. How about reading a hand that hasn't been table? is it OK if there is no action pending or is the player obligated at all times to refrain? At first, rule 60 warns about exposing cards that might influence pending action. And then, it says that folded cards should not be exposed. So, both 59 and 60 make this point.
Besides, without showing the cards, we can never really be sure they are telling the truth anyway! 
True, but the rule clearly says that live or
folded hands should not be disclosed.
When it comes to etiquette, I think is good to have rules that discourage players from pissing off other players, with or without action pending. I agree with Chet, I would give him a warning.
Polite players often tell of the out cards that they folded after the hand has been completed. Why tell someone before the hand is complete that their chances are slim or none?