Love her now, and be ready to help her when she asks. As friends it's most important that we are simply there, with unconditional love, for those who need us. You can't make her want help, or see that she has a problem. You can just love her through the bad times and make sure she knows you are there no matter what.

Let her know that you are always there for her. She need to know that Depression is an illness like diabetes. The is something wrong in your body. If she found out she was a diabetic she would take medicine or see a nutritionist. with depression some people need to see a counselor or take medicine. Please let her know that she is not alone.

It's not that depressed people are not "willing to change anything about the situation they're in", it's just that they can't. They try so hard, but they can't do it alone, and this makes them feel guilty and worthless. Sometimes a severely depressed person can find it incredibly hard to even get out of bed or to go out grocery shopping, especially if depression is accompanied by social anxiety. You can't make her snap out of this situation, cause she can't do it alone. Never tell a depressed person to pull herself together, nor try to analyze their situation (eg. "you know, I think you feel like this because...")Instead of helping her, you may unwillingly push her deeper into depression, and she's going to feel misunderstood, unwanted, unloved and like a burden to everyone. Encourage her to talk about how she feels, but don't try to play the therapist for her, just listen to her empathically and let her know that you're not judging her for her past, and that you understand her. You, the people who care about her (friends, family), must get her to a mental health professional who specializes in treating depression.
As Dearsugar said, getting her to exercise is a great idea. You can also help her with some household chores and make sure she gets good rest. Also, don't let her spend time alone. Try to get her to leave the house, eg. for a short walk.
Try to find a support group for depressed people that she could join, eg. on the internet, or in your hometown.
I think you are wonderful group of friends, she's lucky to have people who care about her so much. And trust me, she knows that, even if she tends to be avoiding or cold with you. Don't take her behavior personal, it's just this nasty illness that depression is, that makes her act the way she does.
Sorry for the length of this post, but I felt like I had to share what I know about this serious condition.

u sound like a reali good friend. i dont know u obviously but i think being there for her is the best thing, and listening. she may seem like shes not willing, but mayb shes just so down shes nearly given up, no one reali wants to b unhappy. i think if u could say that just talking to a conseller would take the weight off her shoulders and that she dosnt hav to go in head first to it but just try a session or two, even tho it may not seem like it, talking to somone hugely helps even if it dosnt stop all ur problems. make sure u dont spend alot of ur own time worrying about it and know that this wont go on for ever, mayb if u ever have any problems url know u always have ur m8 next to u, thats what their for.

i think it says a lot that you are the only one she is trusting with these issues. she probably knows that she is putting a lot of weight on your shoulders...but i'm sure she is grateful to have someone like you in her life. just don't give up on her.

agreed with all above--all you can really do is be there for her. i'd also recommend that YOU talk to someone (even if it's just your mom, or a close friend of yours) without naming names, and let them know how much weight you're carrying.

that said, being up front with her might be harsh. i actually lost a friend once because i told her that she was depressed (she eventually sought help, which makes me happy, but our friendship has never rekindled). i would recommend trying to get her to self-identify by asking leading questions or making leading statements...like, "it seems like you're going through a lot now, how are you really feeling beneath it all?" or "what do you think is making you go behind your boyfriend's back?"

i know it's not the most direct or the quickest way, but sometimes people don't like hearing that you think they're depressed. the way my old friend put it was like, "i have enough going on without you attributing my every move to depression." at least this way you have a chance of getting her to identify her depression as such, as well as giving her a good vent for her feelings in the meantime.

you hang in there, too. good luck, m'dear.
xoxo onesong

There is "absoultely nothing" you can do if she doesn't want to help herself. She knows she is a mess but she does nothing about it. All you can do is say to her you wish she sorts it all out and you would help her but until she gets over herself and starts doing things to change her life you can't make her. You just have to put up with her really. I think that married man issue is so wrong and I'm amazed that you are still her best friend after that.

It sounds like you are a good friend but even a good friend can't change people unless they do it themselves.