Jenny says: It sounds like your friend really needs someone to talk to, and unfortunately, she is making her relationship your problem, too. Part of being a good friend is being there when your friends need you, but on the flip side, it's not fair for her to seek your advice time after time and put you in an awkward position. I imagine it's quite frustrating for you to see your friend unhappy, and your suggestion for her to leave the relationship seems sound.
Sometimes, when friends have problems, the solution appears to be so simple. In this case, your friend is unhappy, and you want her to leave her boyfriend. However, if you've ever been on the receiving end of that advice, you may understand it's not always that easy. Perhaps instead of offering advice, you can help empower your friend. Suggest a self-help book or activities that may give her the confidence to move on. In the end, you cannot do it for her. She has to make that decision on her own. However, given the fact that you have been there for her, I do not think it is harsh to establish a boundary on this topic. Reassure her that you care about her, but also make her aware that you are frustrated with her situation. Sometimes being a great friend is being an honest friend. Good luck!