I was given this advice when I was worried about a friend and the relationship choices that she making....

She won't thank you for interfering in her relationship and despite you only having her welfare in mind it could even force her to choose between you. So be a friend let her get it off her chest and remind her you are there if she needs you. Your going to have to just be patient and wait, be there to help pick up the pieces

Unfortunately several years and relationships later and she was the victim of domestic abuse. All I could do was remind her that she knew what she needed to do to stop the situation but only she could decide if she was ready to leave him.