I've been reading this thread with great interest because a couple years ago, I walked your walk and talked your talk. I know precisely where you're coming from.
My "memoir" is paranormal women's "fiction." It is the story of a close group of friends who were torn apart by the death of one, and how they come to be reunited thirty years later. It was a true story as I wrote it (some of it as it was happening), and I had the very same concerns. We each shared a history and had kept painful secrets. I supplied each of the members of the group with a copy of the manuscript. Some wanted me to use their real names, others asked me to use something different, but similar.
Only one person objected completely. That person was my best friend for those thirty years. Her life is very different now, and she has not come clean with her husband of twenty years. He knows nothing about her past, apparently, and she doesn't want him to. She knew I was writing the story. She filled in blanks for me. She told me of situations I could add to it, including some that were extremely painful for her. One in particular was so poignant and gut wrenching, I knew it would make the story sensational. She said she wasn't worried about her husband finding out because he doesn't read. When the first draft was complete, I sent her a copy. It ended our friendship. Her husband became curious and wanted to read it. She told me she had expected me to bare my soul, but didn't expect me to bare hers. She is so entwined in the story, there is absolutely no way I can take her out of it. I was hurt that she provided so much information, knowing full well what I was doing with it, let me spend two years of my life writing it, and then wanted me to burn it so no one would ever find out. At that point, I discovered I had already lost her friendship, so there was nothing left to lose. Hold that thought, because I'll come back to it.
The love interest in my story is the deceased, and when a person dies, they lose their claim to privacy, so no problem there. Like your friend, he was someone who inspired me, and changed my life and concept of reality forever. His impact was life-altering, and the end result mind-blowing. I know what you mean when you say, "No one would ever believe it's true." I would. I did not write anything about him that I would not write if he were still living. I think that's something you need to ask yourself. If this man means as much to you as you say, if you love him that much, are you willing to incur his wrath, or live with knowing that you hurt him so much, he will never forgive you? Can you live with that? Is that what you do to people who had such a profound impact on your life that you still care this deeply about him thirty years later?
You also need to consider the impact it will have on your own husband, if you have one. Mine knows about my past, but has never been faced with a lot of the intimate details that went into the story. Have you considered that? If you think your story is going to hurt him, you need to sit down with him and do some explaining.
You said your memoir is too long. How long is it? I was there, too, at 222,000 words. Like that would ever get published! I cut characters and subplots, edited, rewrote, edited, rewrote, and somewhere along the way, learned to be a better writer. I learned that a string of events does not a plot make. Does your story have a plot? Do the characters grow and change? Is this guy you're concerned with a Mary Sue character (or whatever we're calling the male equivalent these days)? Have you edited the story? Because a funny thing happens when you start correcting all those things. It becomes less true. It becomes partially fiction (can you say James Frey? I'm almost sure that's what happened with his "memoir.") While trying to get my story down to a manageable size, I started learning a lot about writing. Remember the thought I said to hold? One of the things I learned is to never make a secondary plot character more interesting than the protagonist. I was on the verge of doing just that. The poignant, gut-wrenching truth about my old friend will not make it into the final draft of my story. I know she'll be relieved. I am. I never intended to hurt any of the people in my story. These are people I've loved most in my life.
So, there's more fodder for you to think about. I know it feels like the worst thing in the world to even consider giving up all your hard work. You put your blood, sweat and tears into it, your heart and soul. I would never ask you to give up on it, but would it be the end of the world if you changed a few details and called it fiction? The story would still be told, wouldn't it? And isn't that your point? You want people to be changed by it, and inspired by it, but is there a reason they can't be, if the characters names are changed, and it takes place in a small town instead of a big city, or whatever else you can do to disguise the identity of the man? My advice is to get a couple beta readers, and start editing. You might be surprised how much you can let go of in the editing process. Good luck.