I'd pretend I thought it was a joke b/c as a mom, I could say something like that to my kids and mean it as a joke. (Yes, FIB, I assume the tone she used did not sound like a joke or you would have thought more about it. But this is a small thing.)
I get that you dont' think she meant it as one BUT I swear I've used the words "Dumping your mom just for some ice cream/Justin timberlake tickets? lakers game??" and totally meant it jokingly with my kids I mean who can compete with that? I was fine with it.
And a "walk" is not as fun as catch and she knows this...she really MAY have been trying to make a lame joke). Assuming she was not kidding and was turning it into a contest, must your son know that? If it were not you FIB speaking, I would actually think that you were making a mountain out of a molehill on this one occasion. But I know your history and even if you are a bit too sensitive, given her past actions, I concur with your take for the most part.
Still, by saying "She was kidding" it could let her off the hook without making her look bad b/c this is NOT about "winning" -- but having some peace and it would not have "cost" you anything, but more importantly, it'd reveal how crazy she sounds without being confrontational and starting a fight, AND it protects your son a little longer.
Just a thought.
((( j )))
M: 57 H: 60 M: 35 yrs S30,D28,D19 H off to Alaska 2006 Recon 7/07- 8/08 *2016* X = "ALASKA 2.0" GROUND HOG DAY I File D 10/16 OW DIV 2/26/2018 X marries OW 5/2016