Texting as cheating?

Ignorant and definitely dumb, but infidelity? Hardly.

Thing is the punishment does not fit the crime.
And certainly doesn't justify her having another man in the family home or having their kids with her when she stayed at this other man's house.

Open, you describe it as though he actually had an affair, I believe he apologized and then they carried on.
If the issue had been so great then she should never have said that all was ok.
You don't get to accept the apology and then use their mistake as a stick to beat them with at any given opportunity. You say " this hurts too much I can't do this."
It isn't the what she has done, it's the way that it was done that is questionable. And speaks volumes about her character.

Once again I reiterate, service marraiges are bloody hard work and they require a certain maturity and depth of character.
I totally understand your trying to explain her perspective, but if you want to keep going back over history Have you given any consideration to what pushed him to be texting this other woman in the first place.
He obviously loves his wife, and his family. So what was so wrong that he felt it necessary to reach out to someone else?

You can't make excuses for one side without consideration for the other. And to be honest it could go back and forth indefinitely.

He was away for long periods of time, it's the job, did she write every week even if she couldn't always get a reply. Did she send care packages, send pictures from the kids.
It's hard being married to someone who's job comes first, but she knew what he did and made the commitment anyway.
I don't think she knew how hard it would be or how much it takes to keep things together while they are away.
Or how long it can take for some to adjust back to civilian life.
My point is I don't think this is just about text messages, I think it has more to do with her realising this isn't where she wants to be for the rest of life. That there is more for her out there and she wants the chance to discover it for herself.
Again, there is NOTHING wrong with wanting any of that, but there are ways you do things, and ways you don't.

Dad, hope your hanging in there, If my PM wasn't helpful I apologise.
But do try taking things just a day at a time, and I have a couple of grounding techniques I can share if you feel your anxiety is overwhelming.
Take care.