A friend of mine has a habit of pointing out others' tells to the group that we are playing with (eg, "John just moved his card like so--he definitely has at least a pair"). I argued that this is bad etiquette and akin to cheating. Others said that he is simply sharing strategy and that people do this "all the time" when playing card games. So the question is: Do you tell tells, or no?
It depends whether the hand was in progress or not. I think you are free to tell tells if you like, you will lose out though and will annoy people hoping to exploit this tell.
If this happened in the middle of a hand, however, it's different. In general it is considered bad etiquette and may even be against the rules to talk about the hand in progress. The "one player to a hand" rule means that a player must make a decision alone with no outside input. I think saying something like "you should call, he just gave a tell there" would be against this rule and should be penalised.