A new player has joined one of my informal sessions, and he objects when his opponents warn each other about a shuttle going out, by shouting "out" or "no". So for example he plays a smash, the non-receiving opponent thinks the shuttle is going out, shouts "out" or "no", but the receiver hits it anyway. Our new friend was distracted by the shout, and left the shuttle, perhaps he thought it was his own partner who called "out". The shuttle lands in and our friend loses the point. Two questions: 1. are you allowed to shout "out" before the shuttle lands in your court, and 2. do high level players warn their partners in this way?
this! it's absolutely ok to shout "out" to your partner. if your partner hits the shuttle (because he doesn't believe you or whatever), the rally continues.
Not only is communication with your partner legal, the 3 main calls ("me", "you" & "no") are essential to good doubles. However, be sure to communicate after your opponent's shot and before your partner's. If you call significantly after your partner has already hit it, your opponents can claim that your call is distracting them.
You new friend let himself be unnecessarily distracted. This situation of one person calling it out and then the partner still hitting it happens all the time. 1. Yes, call it out if it looks like it is going out. 2. Yes.
Good communication is essential in doubles. Even when you play a lot together you will always have moments you want to make sure your partner makes room for your smash (MINE) or to know for sure he won't hit back (OUT) or just to "push him" for the last bit of effort (GOGO)