He may use it as an excuse to meet up with OW too... did you put a GPS in his car to keep an eye on him?
It is confidence building to fight an affair once you haev the tools, so I am not surpised how much better you aleady feel.
He didn't tell her on the phone because he needs time to put a sob story together.
The thing is, and you can tell MIL this... he concealed important info from his own parents.. THAT he can't work around ... No matter what he complains that you did.... LYING to YOU, his children, AND his own PARENTS isnt' a solution and he has that on his head now.
That's all she has to say to him when he gives her the sob story :
Husband : ... And there you have it, that's why I am with OW.
MIL : You expect me to beleive that? If that's all it was, my own son would have come to me or his father. You've been lying to all three of us not to mention your children as well... for how long?
Husband : Well...
MIL : End the lies... NOW, we will NEVER accept OW into our lives in ANY way at ANY time... she's attacking your home and YOU are supposed to PROTECT your wife and your home, not hand this predator a key and the combination to the lock... We raised you better than that... You carry your father's name... you should be ashamed of yourself right now... not making excuses.
Husband : You don't understand
MIL : I understnad more than you can imagine... I do have a few years on you ya know... Go home to your wife and start telling the truth...the WHOLE truth... I don't want to hear another word about it.
Something like that would be ideal...
What YOU did doesn't matter... he LIED to everyone. LIES never excuse or solve bad behaviour on the part of another.
