The good news is she seemingly went no contact right away. If true and she is not talking at work then that shows there was no significant dopamine fog happening. Also good news she did not hesitate on a poly when asked. You may have caught this before anything progressed. Not many at SI believe that can happen lol, but of course it does happen sometimes.
The raised eyebrow news, imo. She said "we just talked about work" but she 1) still deleted the texts 2) sent photos of herself and 3) sent a no contact message with I decided to work on my marriage. Those three things do not add up to we just talked about work. Plus add in the look on her face when you dropped POS name to her. Something more was going on. It might not have been physical yet, maybe but surely it was flirting at a minimum, complaining about you too perhaps. I mean, why do those three things if the texts were innocent and about work??
And she said he reached out to me, I didn't look for something. Ok, why did she respond? Why did she send photos? She has some degree of poor boundaries. The book Not Just Friends will be a good resource for you two. It will help her see how she has porous boundaries.
Questions:
Has she confessed what they were actually texting about? Have her timeline detail the answers to those 3 points I mentioned.
Have you tried text recovery on her phone?
Have you looked at the phone records for number of texts between them?
Is the guy married? (How do you know about the domestic violence charge?)
Does she work in a hospital? I ask because the culture at quite a few hospitals is one of flirting, hookups and infidelity. Not all hospitals but something about the setting, hours, pressure etc seems to promote that environment.