If someone is complaining and if you remain silent you are contributing to their complaining. Sometimes we need to vent, but complaining is generally not healthy because it comes from a position of weakness. So tell them: "please stop. You are complaining." Otherwise they may continue to complain about something when they could be putting forth concerted action and real effort in healing the problem by changing it. We can't walk around telling people they are wrong all the time, but if you are present then it is your duty to help them. Being silent helps you, but some encouraging advice may get through to them. Speak from the heart not from the head.
If someone is making inappropriate remarks about a lady; tell them to keep their thoughts to theirself. You don't have to hear it, and you certainly don't have to participate in it. But you can offer a tactful nudge in the right direction. It bothers me when people disrespect women and sadly, oftentimes this is what some women feed on. They like the game of the cocky guy's insults because they are so used to being told by people who they perceive as weaker than them telling them how beautiful they are all the time. So they are attracted to the total opposite of being called beautiful. Instead they find themselves attracted to the "bad guy." They may not even be aware as to why they find themselves being treated poorly or even abused. They just know that they "love" them. So there is a fine line, but if someone is outright disrespecting another person, and they are not playing the game back with them, stand up for them, be the defender of the weak. An example that I recently saw was of someone on a subway getting the snot beat out of them, not a single soul stood up in their defense. Again, for the most part it is best to mind your own business, but if you witness an experience that you know that is unjust, do something about it. Be bold. Even if you don't participate in the fight, stand up and let the attacker know that they are out numbered. But then again on the other hand, perhaps the person being beat needed that experience. Some people need a good ole' punching in the nose. I've had my fair share, and I've delivered my fair share also. But it was in my giving and receiving of bloody noses that lead me to where I am today. Everyone fantasizes about being a hero at one point, but sometimes the hero is infact the person delivering the punch. So just use your best judgement when and when not to get involved, but also know that sometimes people are truly innocent and need defending, and other times they may desperately need that beating to change their life. Some of the best lessons in life are delivered by the physical exchange of energy. We are not to be the scourge of the world, but sometimes the scourge of the world learns through those experiences and discovers the warmth of compassion. But they never would have gotten to that path without being a tough guy for many years.
Being silent is often the best method because you remain within your strength, but also being silent does not always help others learn and sometimes a powerful word can help set someone straight. Silence is usually the best option, but if you love someone you will try and help them see why what they are doing is not healthy. Do not infringe upon others. Allow them to be as stupid and silly as they need to be, because their life is sacred and we can never fully understand anything but our own lives. But a word of compassion can help point the way, it will be up to them to walk the path. And when they walk that path there may be some physical lessons to learn along the way, and a punch in the nose teaches a boy what it means to become a man: to accept full responsibility for their actions.
Some actions might just deserve a punch in the nose. =P