Originally Posted by Spock
So this goes back to my wall of text.
She didn't screw up by falling for him. If she screwed up it's because she is failing to devote enough attention and love to you.
Then ask her to devote more love and attention to you.
If she can't handle falling in love, she shouldn't have a FWB. If she can handle falling in love, then she needs to make sure she behaves responsibly and dedicate sufficient love and attention to your relationship to her satisfaction. If that isn't enough for you, you have to ask for more (which she might not be able to give!). If she cannot give you enough no matter how much you ask, you need to figure out why you are asking for more than she can give.
I bring this up because you brought up her lover as an escape; escape from you?
^^ +1 to all of this.
Basically, you and your wife need to talk, face to face, and get some things straightened out. You said that you were threatened and that she was freaked out. But big picture, are YOU okay with her loving another and is SHE okay loving another? If you are both okay with that setup (big picture/conceptually), what will you need from her in order to make sure you dont feel threatened by that other relationship. What will she need to not be so freaked out (if anything)?
Communicate those needs to each other and then you will know where you are and hopefully have an idea about where to go.