Depending on the legalities and financials she needs to know.

Better do it now.

Rather odd for 2 people of this age to divorce.

I have always believed if you have affair info, or divorce info---talk to the person who is doing it, "Hey I know you are blah blah blah, either you tell MY mom today, or I will"

Giving the person the chance to come clean to the spouse first.

This is awful for a woman of her age. Get a good lawyer for her.

Very sad, I am sorry.

Sometimes, it's better to stay quiet. While what you said may have been truthful, is this the outcome you wanted to acheive? Was it helpful or kind?

You don't say anything about your mother's feelings towards her husband, but your words should have been chosen with consideration for her. Your SF may be an a$$hole, but he's HER a$$hole and it's her choice whether to stay with him. How do you think she'll take the news that he's leaving?

I have a brother who sends around all kinds of nonsense. He's an anti-vaxxer, so 'nuff said.

I just delete it. I'm not going to change his mind.

You've been a "member" here for less than 2 hours and you want advice on how to deal with a stepfather you don't like.

As advertised, this is a site for STEPPARENTS to vent.  Typically, stepkids (no matter their age) who come on here asking for advice on how to deal with a stepparent are usually not going to get the kind of answers they want, Not to be harsh, but I don't understand what kind of insights you want from us.  

It's kind of like someone going to a support group for battered spouses.  He's a stranger, shows up for the first time, sits down and then tells the group he likes to take swings at his wife because she is so annoying ... and then expecting the group to support that viewpoint.  Not going to go over well. 

Idk what you guys situation is, but from a stepparent point of view, a son living at home at age 43 to "take care of his mom" would be a worst nightmare scenario. He may ne the devil himself for all i know, so if your goal is to run him off, keep doing what you're doing. Keep living there, keep publicly correcting him, and keep involving yourself in their relationship. If you think your mother would be happiest if her marriage is stable, please stay out of it, move out, and leave them to their final years and visit and talk on the phone. 

You've lived with your stepfather for 25 years, so you've never actually separated yourself from your mother to build your own path and individual life? 

You're 43 years old, whining because stepdaddy writes books about himself. People do that; it's called an autobiography. How many people forward BS on social media? Millions! You sound like a petty 43 yo mama's boy who can't stand having to share her. Stepdad is probably beyond sick of having his grown stepson under his roof. 

You mentioned that your mom detests being around him, so why hasn't she left? This is HER relationship, HER marriage, HER business. Yes, you should feel guilty about breaking up her marriage. Mind your own business! You keep referring to "us" and "them". There should be no "us" or "them". The marriage belongs to two people, not four. "I'm almost wanting to say GTFO before he's due to leave in a week so I can beat him to the punch". Your attitude is that of the stereotypical entitled bratty stepkid. Grow up man. It isn't your place to determine what's best for their marriage. I guarantee you've played a large role in any bad attitude your stepdad has. I love my stepkids dearly, but there's no way in hell they'd be a permanent fixture in my home after 20.  25+ years?? As a stepparent, if my husband refused to give his kids the boot, I'd leave them to each other. 

If this man, your step-dad, was indeed a “malignant narcissist, veteran,” he never would have been magnanimous enough to admit it to you, much less to all his other contacts.  Narcissists never admit they are wrong to anyone, including themselves.  This man not only admitted he was wrong to you, but further noted “his error” to his contacts.  I’m guessing he could have easily countered back, but out of generosity went along with your disagreeing with him (a/k/a wanting to stick it to him) to keep the peace.  THAT is what his kids were probably PO’d about—you asininely trying to one-up step-dad, probably for the 10,000th time, and rub everyone’s noses in it.  

It is not uncommon for narcissists to think others have the problems that they do.  Here is a list of characteristics:

1. He is melodramatic:
2. He is the croaking frog of his small well:
3. They often have a history of abuse or ignorance:
4. You are either a delicacy or a toothache to them:
5. Their head is always clouded in superiority:
6. They are selfish:
7. They can’t empathize with others:
8. Can be enslaved with flattery.
9. They dislike most of the people.

Now I can’t say what characteristics here your step-dad may or may not have.  However, since he did admit he was wrong to you and others, he at least appears to be able to empathize.  You came to a site supposedly looking for advice, and when you were given it, you stated, “It's very disheartening to see the animosity on this site without knowing the situation.”  To me, that at least hints of lack of empathy or feelings of superiority.  But, you did like Dc3sc2’s advice, perhaps showing you “can be enslaved with flattery.”  I would even go further and say it sounds like mom is your delicacy and step-dad is your toothache.

Facts matter.