What to do with tough situation?

I have been friends with someone for a long time. I was there for her for most of her boyfriends and she was there for most of my girlfriends. We lost touch for a while, then one day I found her. She is married and I was engaged. My engagement broke off soon after wards and she started telling me about her and her husband having problems. We have gone out together with our significant others and without. Nothing has happened, though. Now I am getting the feeling like that might be changing. She asked me to have her kid when she gets ready. She won't tell me if her husband knows about it or not. I am starting to have feelings for her, but don't want to act as long as she is married. We have been friends for so long and I don't want to turn my back on her if she needs me.