Yes, you can point out that the opponent is foot faulting in America, but unless you can get an official on court that's about it.
But, really, you are not in the right position to make such a judgement and if there is no real benefit to the server it's really a question of your ethics.
The rules for non-officiated tennis are slightly different for good reason, as this thread has amply demonstrated.
Originally Posted by tennis tom
Whatever semantic gamesmanship you want to play with the FF rule here, one is certainly within their rights and the rules to point it out to the opponent that he is FF'ing. What the perp does after that is up to him and his ethics. If being called on it when he does it, gets into his head, it's his fault for FF'ing and giving his opponent the opportunity to get into his thick head. You may not want to ever play again with someone who calls you on it, but he may not want to play with you again either.
All the things you mention are in the specified rules of tennis or rules for guests and members of tennis clubs and facilities. If you don't like someone getting into your head, then follow the simple rules of the game and don't give them any ammunition--they are so simple to understand and follow. If you want to question authority buy a skateboard.