UPDATE:
So I waited at the café for her to arrive. The place was packed with people like I hoped it would be. She was about 45 minutes late of course. She came in all huffing and flustered. She wasn't in a good mood. I am sure she rather would have been with her boyfriend or somewhere else. She sat down and I calmly asked her how her day was, and then she launched into the usual marital daily debriefing, the same format as every daily update we gave each other over the past two decades. When the waitress came she ordered her usual and I just ordered a refill on my coffee.
SHE: "Why didn't you order something to eat?"
ME: "Because I don't plan on being here much longer."
She looked at me like I had grown a second head.
HER: "What's wrong? Look I'm sorry I am late but you know how things get at the office."
ME: "M_____ I want you to listen and not interrupt what I am going to tell you. M_____ I know you have been having an affair with R_____ for some time now. I have known for two months. I have seen a lawyer and he will be having a divorce petition served to you next week. I would like to do it somewhere outside of your office to spare you any embarrassment or cause you problems. So when he tells me it is ready I will text you with the service location and time."
She sat for a moment, hen a shudder went through her, then she took a deep breath and laid her head in her hands.
HER: "Oh God."
ME: "No, don't bring Him into it. This had nothing to do with God. We dragged God through this the last time you did this and I won't do it again.
HER: P____ please let me explain, I...."
ME: "No, I am not here for that. I don't want that. Look, I know probably as much about your affair as you would ever be willing to admit to me. I have months of dates and evidence. I don't want to know your reasons or your justifications. I just want to both have some peace. Neither of us has been in a real marriage for fifteen years. We have been spinning our wheels and I know you are not happy and that you have not been happy for a long, long time. And I'm just tired. I'm tired of this marriage and I want it to end so I can go on with my life and have some peace."
Tears were running down her face and she was sitting back with her arms crossed just staring into nothing.
HER: "P_____ I'm sorry."
ME: "Look this needs to end. I don't want apologies. Apologies are meaningless now. I just want a fair and equitable divorce. I want you to get a lawyer and have him look over the petition when you get it. I'm not going to try to railroad you M_____. I don't intend to embarrass you publicly. I want this to go smoothly and I don't want us to fight. Right now I feel nothing towards you or this marriage. Please don't do anything to make me hate you."
She sat there for a good minute just staring and quietly crying.
HER: "What about S____?" (Our son)
ME: "I will call him. I"ll tell him I am divorcing you because of infidelity. I won't lambast you or get into any details, but I am not going to lie to him M_____. He will want to know why. He is an adult and he can absorb it. It will be up to you to repair whatever breaks between you and him."
She just nodded. We just sat there for a long time, saying nothing. She looked at me after a few moments and I could see there was some deep emotion in her eyes. There was still some love in there somewhere. She was a mess by this point and she couldn't stop crying.
That's when I decided to leave. I told her that I was going to be away for the weekend to give us both some space. I told her I was going to call our son and then after that call I was turning off my phone. I told her that our parents would need to know and that she could tell hers and I would tell mine. I told her I had no intention of letting anyone else know about her affair outside our immediate families. She just nodded, and I grabbed her hand and held it for a long time. She squeezed my fingers hard, almost making me wince. I have to say I was surprised at how sad she was.
Then I just told her bye and I got up and left. I had a bag packed and I drove to a hotel on the coast and stayed there through last night. I called my son while I was driving and he, surprisingly, took it better than I thought. He said he could feel something was off with me over the past few weeks and figured it had something to do with his mom. Then he laid a bombshell on me. He asked me if she cheated on me again. I said "Yes... what do you mean again?" Turns out he knew about his mom's first affair. He found out about it from his aunt who accidently shot her mouth off and spilled the beans to him about three years ago. He never said anything! Poor kid. I had no idea he was carrying that burden. It pissed me off. I was doing well until he told me that. So I just spent Friday night stewing.
So that's it. Sorry if I wasn't able to give a more dramatic story, but it really went off without a hitch. Very anticlimactic, but I guess that is a good thing.