Do you think it's acceptable to intrude into a friend's business if you can see that they are in a relationship that they should get out of? What does it depend on? And, How would you feel if a friend tried to do that for you?
Unless the relationship is with a murderer or rapist etc, then I'd say leave it alone if you've talked to your friend about it and they're still in the relationship.
I think the key is how you do it. If something happens and they are bothered by it, you should help them think about and organize their feelings. Don't just tell them what to do - they'll need to decide that for themselves.
Try: "So that really happened?" "Why do you think ___ did that?" "It sounds like this is bothering you a lot" "Well, if you could change things, what would you change?"
Also, people get stuck in relationships because of self image issues. Maybe they think they deserve the abuse or maybe they think they don't really deserve any better. Or, maybe they are scared they won't find anyone else. Reinforce their self confidence with compliments and supportive statements:
"You deserve better than ____" "You know, I always like talking with you" "You look really good in that shirt/suit/outfit/dress, I think I noticed that girl/guy looking at you"
If your friend has asked you for help, help them. Otherwise, you're just going to turn friends to enemies.
I said no. WHIT THE EXCEPTION that if that friend is in danger ie physical, emotion, or sexual abuse. In these cases try to help. Most peopl will deny help if they are "In love" but it is for their own saftey. I would want help if i was blinded like that.