I never should have but I did... Now what?

[i_Am_That_Guy]

I was in a new life program just about to graduate and was asked by my Case manager/friend if I would like to be interviewed to help out one of his classmates. When she came through the door I was speechless. She was beautiful and I had to know her. Interview went well I was interested and I told my friend I had to see her again. I bugged him day to day and finally he said okay damn! When you graduate I'll hook you up....
Graduation day came and so did a text from a strange number... I was excited when I found out it was her. We talked and texted late in the night every night and want long before we met up. I was sprung on her and she was very interested in me too.
We started dating and things were fresh and very exciting we never fought always looked forward to next time we saw each other. We were in love.
One problem.... She was married. He was in prison and they had never physically been together. She didn't know what she was getting into and had cheated on him before. I used that as justification and told myself their relationship was tainted and she should have a fresh start with me.
She started talking to him less and getting closer to me but when the season changed so did her heart. She would write him or buy phone time and hide it and lie but I knew every time. He knew about me and wanted me gone but I fought hard to have her as my own. I won every time. Lots of tears and respect and trust were lost during our first year together after going back and forth she finally got a divorce.
It didn't change anything like I thought it would. It made her resentful and she lost respect for me. I understand that now and told her I'm sorry I interfered with what she had before me but that I'm not sorry I got to meet her.
It's been another year and a half almost 2 and no contact from the ex husband. We are engaged she has the ring we designed on her finger and mine is being built right now. We have plans of being married this month. He called 2 days ago said he's getting out and will be paroling to our town rather than out of state like we had thought. She said he asked for some help. Towel, soap, etc. I call bs but before that I say absolutely not! Am I afraid of him causing physical harm? No. No am I threatened by him? Absolutely. She said she doesn't expect me to understand or be ok with it and is going to help him. They have no kids and are not married so why? I told her I'm not at all ok with it and that I won't have it. She would give up what we have to help him. She says he told her they are strictly friends and he has no intentions of interfering with my relationship with her. I know she has feelings for him and curiosities that she has only ever dreamed of.
What do I do?
We both have 8 year old boys that know about our wedding plans and already cal each other brother. We live together by things together accounts together. We have a life together and plans together. I know they once did too and regardless of what he says I think the worst.