Here's more backstory:

xMIL and Mr. A's dad met when they were young. xMIL was Mr. A's age (or younger), and Mr. A's dad was my age (or younger). They were together for a number of years and had 2 kids (Mr. A and his older brother). Married right away.

Then Mr. A's dad did this:

1) Called xMIL at work and told her they had to talk that night. She was worried.
2) Told her that night that he was in love with someone else and was leaving and never coming back.
3) Left and took all his things within the week.
4) Was officially divorced from her within a month (so she says).
5) Married the OW but did not have kids with her.
6) Treated Mr. A and his bro like [censored] through the rest of their childhoods and young adulthoods.
7) Mr. A finally excommunicated his biological dad in 2002: "Dad, I never want to see or hear from you again."
8) Mr. A decided to get back in touch with his dad before he left me in April 2009. I have never met Mr. A's dad.
9) I don't know how that all turned out, but it was off to a rocky start. Dad was not very nice.