Let's do a bit of role playing so that people can see what went wrong and where!
What I'm assuming the conversation went like:
Phone rings...
WS Husband: "Hello?"
Ex-Wife: "hey. Do you mind taking the kids over Christmas break?"
WS Husband: "No problem. WS is off work anyways."
Ex-Wife: "thanks"
What the conversation SHOULD have looked like:
WH Husband: "Hello?"
Ex-Wife: "Hey, do you mind taking the kids over Christmas break?"
WS Husband: "
I don't mind, but let me check with WS because I think she might have made plans to go out of town."
Ex-Wife: "ok, just let me know soon or I'll make other arrangements"
(now, the ex-wife could have responded in a total crappy, manipulative way about him not loving his kids, etc - but we'll give her the benefit of the doubt and assume she's normal and sane)
WS and her husband have a conversation and she communicates that she is ok with watching the kids but that she expects him to contribute, etc. She explains her boundaries UP FRONT.
This is what normal, healthy, communicative relationships would look like.