My take on the conversation, I'm remembering what was said so it's not exact. And I'm making up the name Fred.
Her
Hi Fred , how are you? I've been thinking of you over this last crazy few months. We had some good times together.
Him
It's nice to be remembered of so well. Thank you.
Her
I always think of you well, do you think well of me?
Him
Yes
Her
In the same way I do?
Him
Yes
Her
Things could have been so different
Him
It was my fault
Her
What about your wife?, It wasn't all your fault, I was very hurt though.
Him
I am married now yes, and we can't go back, but I am very sorry for hurting you.
Her
But I'm single now, I thought maybe we could meet up?
Him
No, that's not going to happen.
So she sends the screenshot to stir up trouble, he feels guilty for getting involved with the conversation, now he realises his ex is a manipulative woman and his wife is angry at him. Hence the bad mood.
Just my take. He's not innocent, but it's far from unforgivable.