It’s really hard to watch your friend get hurt by their boyfriend or girlfriend time and time again and not step in and offer some advice. Some friends may be open to and welcome your input. Test the waters; but if your friend seems upset by what you are saying or is shutting down, back off! The most impactful thing you can do is help your friend realize for himself that this is not a healthy relationship.
There are lots of reasons why your friend is dating a jerk to begin with. Maybe he is afraid if he leaves, he will never find someone else. Maybe her self esteem is really low and she feels like this is the best she deserves, or maybe he really does love his girlfriend and is willing to put up with the bad things in the relationship.
The best thing you can do for your friend if he/she is unwilling to listen to advice is to be an active listener. If you become the person who listens, they will talk and open up to you, rather than closing off and tuning you out. As your friend talks, ask questions. Ask them things like “Why do you want to stay with him?” “What would the next year look like if you stayed with her?” “What would it be like if you got married to her?” “How do you think you should be treated in this relationship?”
Offer encouraging support. Phrases like “That must be so difficult for you.” or “I don’t know how you’re able to make good grades and deal with all this” really show that you empathize with them and will encourage them to continue conversation, rather than shut them down.
If they are able to recognize their relationship’s negative qualities themselves and say it out loud, it is much more impactful than you telling them what’s wrong with their partner, or their relationship.
If they do end up breaking up, you can help them go through the grieving process by continuing to offer a listening ear, but also by spending time with your friend so the “empty” place is filled by activities and friends and he/she isn’t tempted to get sucked back into the relationship.