I think this is a case where you might want to consider being "cruel to be kind". If I were you, I would explain my observations about my friend to her, and say that what you think she needs--far more than continuing a relationship of any kind with you--is to find a good counselor. [She might take this badly.] Then emphasize how important your marriage and wife are to you and that you are going to continue to strengthen that bond by not seeing your friend.

I would not protract the conversation. I would also make a conscious effort to find friends of my own gender, and couples that my spouse and I could befriend as a couple.

This might not be what you want to hear. You might grieve losing a friend, particularly one you have known for so long. But the longer I live, the more I realize that some mistakes can have far-reaching consequences if we hesitate too long in correcting them. Certainly your friend needs friends; she may want and desire a love relationship. But at this point in your life you should not be fulfilling those roles for her, given the circumstances. The best way you can be a friend to her is to encourage her to find a healthful life for herself -- and counseling is the best way for her to discover that, in my opinion.