Have you ever watched the younger generation w/their phones and texting? The equipment is attached to their ears and their fingers. She is 21 and that generation is all about texting. You can't be sure that she is the only one that he is texting. He may have a number of new friends that he's in communication with.
Yes, the crisis is all about emotions and right now, he's gone back in time to that "younger" age to find what he thinks he missed out on.
Whether it is an emotional or physical affair, they both have to die a slow death and at their hands. Right now, she's stroking his ego and showing him a good time and there is no history w/them, i.e., only the present. You represent responsibility, accountability and "history". You know him like a book and can spot when he's not himself. But, some time in the future, he may turn back to you for that safe place to land. Time will tell on that and when it will happen.
The one thing that you must remind yourself each and every time you compare yourself to the girl is that you are the prize. She will never be able to replace you because you have the wisdom and strength that has held your family together. She doesn't have that w/him, i.e., it's just play time. Again, as a repeated reminder, this is all about him...not about you and you need to remember this.
Keep the focus on you and your family. Life is far too short not to live each day to the fullest.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.