Cheating with another man's soon to be wife.

I'm not really sure what to do in this situation. I'm caught up with this woman who is also a mom. The man she is with, has almost gotten physical with her in the wrong way. She always calls me (4-8 times per day) telling me how mean he is being to her and how they fight all the time and how she doesn't want to be with him, she says she is just with him for the kids at this point. I know i have some real feelings forming and dont reallyknow where to go. If i tell anyone i know word might get back to the spouse. We have been physical (yes we have had intercourse on more than one occasion) she tells me that I'm thebest she has ever had and she will do anything to see me. At this point i don't have to many responsibilities as far as kids or animals. So I'm always available for her to come over or me go over to her house. She weights about 140lbs on a bad day, i know because i am also her personal trainer. Her soon to be husband is about 380lbs on a good day and gaining. She is fed up with how lazy he is, how he doesn't respect her or let her even have a bank account. He is controlling and has no joy or want to please her. He wants to drink watch football and not plan anything with or for her. She wants out of this relationship really bad but seems to not be able to let go. What should I do? Leave this almost married woman in a fucked up controlling potentially dangerous situation? Take her from it? Or keep playing this cheating game until we get caught? Let me tell you, I am over there hanging out or we are going to the gym 75% of the time throughout the week. He has to have an idea or be somewhat suspicious us spending so much time together... Please share your thoughts and opinions on either what you think about it or what you think I should do is.
Thank you,