It's maybe a violation of one of their
site rules:
5. Players and observers may not discuss a hand until the action is complete. Discussing cards discarded or hand possibilities is not allowed.
If it's a player that's actually
in the hand at the time though I'd be surprised if any action was taken. Who's to say he's telling the truth anyway? Giving your opponent misinformation about the strength of your hand is such an integral part of poker and I don't think anyone would take this as a serious violation of the rules.
A player that's folded saying that they would have had a straight, whether it's true or not, is a completely different matter.