Would I be allowed to say to my friend at the table "I bet 10 dollars that gentleman there wins this hand." Assuming that both of us are not currently in the hand.
Is this rude? Is this legal?
I like Chris's comment as the answer. I would say no, you cannot. It is tacky and rude. It is not a bet within the scope of the casino's license, so technically it is an illegal bet. To make the bet you would have to discuss the hand in progress in such a way as to violate the rule about discussing the hand in progress.
It's poor sportsmanship, and almost certainly against casino rules.
To make that remark, you would probably have seen someone's hand, or if a skilled poker player, observed someone's body language or "tell." In any event, by making a bet, you are commenting on a game in progress. This is forbidden to both players and spectators.