EA then PA with her ex

[Anaru]

At the end of 2 week business + personal trip.
Felt good connecting with family and old friends - reminding myself what it is like to be treated as someone worthwhile.
W visited OM a couple of times while I have been away.
We discussed her writing a letter - she though it was a good idea as she just could not express herself to the guy.
So she did that then I get a message that she went to see him anyway. She got back around 0100 am and messaged to let me know she was home. They talked had dinner, walked the dogs and went the sauna (!)
I did not react immediately as was out with friends here (morningtime where I am). 30 min later another message assuming I was angry as no immediate reaction my side and that she was turning her phone off.
When I saw the messages I just said neutrally was out with friends and we would talk further when I got home.

Will assume she is done now and insist now on NC or she moves out.
Mulling over whether to discuss the situation with our sons (20, 18, 14) - any advice on how to discuss this with teenage/young adult sons?