If you want to maintain the relationship, I would ask your wife to help you? I would make it a more steady, 3-legged-stool, relationship. The friend sounds like she could use a woman friend too and, if/when she is in her next relationship, you can see if she then is able to make it a couples' friendship thing.

My husband was married before and his ex-wife and I are friends. No, we do not do things, just the two of us (she lives a couple hours from us) but we all get together for holidays, family parties, etc. I help my husband know how to relate to her now, help keep things pleasant for my stepsons, grandchildren, etc. If you want your friend to remain part of your life, I would ask your wife if she could make her part of your life as a social couple.

I do not know what you are sharing with your friend that you do not share with your wife? I do not think having two "compartments", one "friend" and one "wife" could work for me. My husband is my best friend and the rest flows out of that.