Several theories here:
1. His daughter told him she couldn't stand this woman.

2. He came across someone who really knocked his socks off.

3. Ex wife came back.

4. Ex wife put up a fuss abut having this woman around the daughter for whatever reason.

It's possible the gone guy wasn't aware that the woman in question was that far into the relationship and just dropping out would be hurtful.
Some guys don't think they're all that much and that there's no reason to think anybody would be really attached. As if they're only as good as the last date, or possibly the next one.

Sorry Bartak and Richard. I call bullshit on you guys. If you have been seeing a woman for three months and have even introduced her to your daughter, you should have the balls to tell her you are no longer interested, no matter what the reason.

I can't imaging just disappearing after being involved with someone for any amount of time. Sure it is uncomfortable, but it is what an adult with character does; versus the running away from responsibility that a child does.

Did he introduce her to the daughter, or did the daughter happen to be around one day?

for someone I'd only been dating a little while, I feel like I'd prefer ghosting. I don't know why everyone says "They deserve to hear it face to face!" The last thing I deserve is having you witness the humilation on my face while you're dumping me.

"But to cry in front of you-oooooo
that's the worst thing I could dooooooo"

I've never felt a relationship needs to be ended face to face, but I do understand the 'tell me why so I can figure out if I need to fix something about me' argument

I had a number or relationship end, finally one girl told me I was an "unrelenting asshole" when correcting people in an argument.

And that is an entirely fair characterization, so going forward I took better care to explain to those I started a relationship with that as some who literally almost died ten times before I was twenty, as well as a few other issues, I have very little empathy so it would be best NOT to discuss certain issues with me as I have no patience for making stupid people feel better about their stupidity when they try and make their stupid emotions based logic defunct arguments on how the world should work. Life is precious and I dont intend to waste mine making sure your friends never get their feelings hurt for being factually wrong

And its made my subsequent relationships smoother.

Jay. I was a fraternity grad adviser back in the day. Most houses back then had actual house mothers. We were different. I was the meanest mother on campus.
Talked to a lot of people who thought I knew stuff. One thing I learned that a lot of guys, even those with a good front, really did not think they were valuable or could be valuable to a woman. They figured that whatever they had going on was the result of their effort, not a matter of the woman in question really being into them. That could not be imagined and so ambiguous or even not so ambiguous stimuli--as the pshrinks call them--could not be interpreted as meaning the woman was really attached.
So if somebody more attractive appeared in their world-which would include being amenable to their company--going there wasn't costing the other woman much, since she'd been more or less putting up with him.
This is not a perfect answer but it must be considered. Some guys are jerks, some guys have no clue that they are valued, some guys have some of each.
Consider; to think a woman really, really likes you requires you to think yourself really, really, or at least some, likeable. Valuable. Desirable. Why would a guy think that?
Plenty of room for confusion.
Sort of related, some women also thought I knew stuff and I had to listen to some asking what they had to do to get so-and-so's attention. My point is that so-and-so wasn't getting it because he didn't think--imagine, feel subconsciously--there was anything to him to make him valuable to women in general or to her in particular.

I read things like " I eventually texted him" and have to wonder how one sided was this relationship?

Three months? Something happened.

Besides the first 4 reasons, perhaps she showed some aspect of herself that really set the alarm bells blaring. Something important like 'When are you going to pay off my student loans' or 'you realized after we get married, I am no longer blowing you.'

And if SHE is, ahem, drama prone, who has a tendency to go into huge weepy displays, historonics, abuse (yes, girls do it too), or yelling jags, why would he put himself through that just to spare the feelings of someone he is dumping?

As Amy pointed out in passing: it is a full on gender neutral thing. Actually, no...girls do it a LOT, since they tend to be more chary of confrontation. "Tell him I'm not home". Date no-shows. Didn't Huey Lewis do an entire song about that very tendency?

So if this guy has been ghosted or shined on a few times, why should he feel any compunction about doing the same to her?

We used to have nice rules of courtship but they have been tossed out. Welcome to the new world, where we can all be equally ghosted.