One year ago when my W was caught up in her A, I thought the same as you. I felt powerless. She was infatuated with him, she didn't care about me, so what did I have to use as leverage?
I've since realized that she wasn't nearly as certain and confident as I thought. When I'd take a hard stand, she'd start to doubt herself. When she saw me weaken, her strength multiplied. IMO, you do need to be willing to put the M on the line. That's your leverage.
As an example, my W told me she didn't love me, she told me our M was over, she told me to let it go, she told me she was moving out. Yet after I found out about her A, I told her I was considering filing for D, and she got very angry and shouted "If you D me, I'll never forgive you!" Her reaction threw me off balance, because I wasn't expecting it. After everything she said, why would she care if I divorced her? Because she knew she was way out on a limb, and she wanted to control everything. The threat of anything getting out of her control made her panic. The next day, she was in the strangest mood, and was flirting with me, and told me she didn't like the thought of me being with someone else. Again, threw me off balance because it seemed so incongruent with her other actions.
Moral of the story, refuse to let him be in control. Remove his sources of security. Make him see that if he's going down this path, he's doing it alone, and he will have to suffer ALL the consequences.
