As an outsider, it's easy to locate discrepancies in the relationships of others, especially our friends. But is it right to interfere? We must remember that no one is perfect; we all have our flaws. We all have our highs and lows. We must also remember that it's impossible to know what truly goes on between two other people, besides what we are told. There is that intimacy in relationships, those feelings which we cannot see as outsiders. Even if, at face value, a friend's relationship seems unusual or uncanny, it is impossible to know the deepest feelings of both parties.
There is nothing wrong with giving our friends feedback about what we observe. If something seems wrong or out-of-place, it's okay to talk to our friends about it. The friend may share new insight about their partner which can skew our judgment or make us reconsider our comments. That friend may also reveal that they are not as happy as they may seem. But don't confuse venting as a cry for help. Everyone needs to vent sometimes, to complain about their relationship. This doesn't mean that they're necessarily in an unhealthy relationship. It is natural to want to protect our friends, but it's probably not a good idea to step between them unless your friend is in actual danger.
In situations where a friend is in danger, the rules change. Abuse comes in many forms: physical, verbal, emotional None of these are okay. And unfortunately, many people with abusive partners stay with their partners, either blinded by love or by fear. They may not realize they are being abused. In cases of abuse, talk to your friend first. They may be grateful that you're there for them, or they could be angry with you for "judging" them or for your interference. Remember that they are still in the relationship with this person for a reason. It is possible to continue to love someone even after they have endangered you or abused you. Offer to help in any way that you can. Be careful not to endanger yourself or further endanger your friend. In very serious cases, involve the law. Contact the police.
Again, everyone has their highs and their lows. Unless your friend is in danger, it is best not to get involved in other people's relationships. If you disapprove of something you observe in your friend's life, it's okay to talk to them about it, but it's a good idea to put yourself in their shoes. If you're in a relationship with someone you truly care about and your friends constantly point out all the negatives, you're bound to get either annoyed, insulted, or hurt.