Regardless of the cause, your main concern is that someone you love is not being respected or appreciated.
It's painful to see anyone you care for in a relationship that seems to have a negative effect on them. You want to say something, anything. Unfortunately, the person you care about may not be ready to hear what you have to say, and it's hard to know if you'll regret saying something, or if you'll regret saying nothing.
Relationships are a complex interplay of deeper dynamics too extensive to discuss, but let's touch on some basic points that can help you navigate this delicate situation without alienating your loved one when they need you most.
1. Be compassionate.
Understand and acknowledge that we all are imperfect human beings. We make mistakes, even in our judgments of people. Emotions, interpretations, and the experience of "love" are not logical. The person you care about may be "erring" in judgment in your opinion, but you've been there too; perhaps in a similar way or many other ways. So relax a little, and step back with your harsh judgments.
2. Recognize their right to make their own mistakes.
We want to protect the people we care about. But we have to let others learn, grow, and make mistakes on their own. As difficult as it may be to watch, it's their lesson to learn. They may “see the light" or they may not. It's their life and journey, not yours. Accept their path. If you love and care about someone, respect their choices, even if you don't agree with them.
Voice your concerns in a reasonable way. Be wary of making this person feel attacked. Have an honest conversation about your concerns, opinions, and observations. Resist the temptation to exaggerate or impose your feelings on their relationship. It should be a logical conversation based on facts.
One conversation is more than enough. A million repetitive statements will do nothing other than belittle and alienate the person you care about.
It's important to avoid attacking their significant other. Doing this will risk putting your loved one on the defensive and potentially push him or her deeper into that relationship.
4. Don't distance yourself.
This may be subtle at first and a natural tendency, but don’t change the nature of your relationship just because you dislike their partner. You might not like him or her, but you have to accept and tolerate that person.
If you really care about someone, then your personal relationship with that person should be prioritized above your dislike of their partner or their relationship. After all, that could end tomorrow. You were there before, and you'll be there long after. You could be their lifeline back to sanity, so don't cut the rope.
When they do get hurt or heartbroken or taken advantage of, refrain from saying "I told you so" or "See what I mean?" It's time to listen, not judge. As angry as you may feel in that moment, be the calm confidante that your loved one needs.
If you see unhealthy patterns continuing from relationship to relationship, it's OK to point that out gently and make further suggestions like therapy, to encourage them to get to the root of the problem. But it's about timing and delivery.
Most importantly, make sure the person in question knows that your love is unconditional, and you will love them despite the mistakes they make. That is what real love is all about, and that is beautiful. Hold onto it and don’t let it go.