It pains to see someone you care about stay in a bad relationship. You feel that if she would just let go of him her life would be so much better, and it's difficult for you to understand why she would stay. You can help your friend let go of a bad relationship by gently helping her to change her thoughts while supporting her emotionally.
A person's thoughts dictate their beliefs about a relationship, and confronting these thoughts is the best way to get someone to let go of destructive ties.
Encourage your friend when she talks about life without her partner. Remind her how amusing things were before she began her troubled relationship and that you would be there to support her during the breakup.
Re frame your friend's negative talk about ending the relationship to positive talk by letting her know that the breakup would be a time to recapture lost friendships, interests and discover new talents.
Ask your friend to move in with you if she is living with his current partner. Your friend will know that she has a safe place to stay and will have emotional support until she gets back on his feet again.
Encourage your friend to see a counselor if her relationship is abusive.