My long time friend has recently confided in me of the marital problems she has been having. I suggested that they enter counseling, to which she replied she had mentioned it to her husband, to no avail. I then urged her to go by herself, and she said she knows what the problems are and deals with them just fine.
Is there anything else I can do to help her, or have I done all a friend should do?
You have done all you can. If you aren't careful then anything that happens after this will somehow be your fault (no, I know it's not), and if your friend doesn't want to consult a professional then there's nothing you can do.
Be a friend to her...it is a long road from marital problems to either a livable solution or divorce. She will need a friend along the way. And no, you can't do much more than listen most of the time, but you have no idea how much that means to the person going thru this.
Of course it would be wonderful if they could go to counseling. However, in my limited world experience, most men say yes to counseling only when they think it is the only option left to stay married. And by then, the woman is done.