I would just quibble that I did let my EW know that I was going out of town, and the exact dates I would be gone because that only seemed fair, and I didn't want her to come to the house and not find the dog. After she screwed me over, I found another place for the dog - I've done that a bunch, generally through paid dog-sitting services, and never told her where.
The way I look at it though, you're divorced. The mutual friend apparently talks to both of you, and probably assumes there is no longer any bad blood between you since the D is final, and probably saw no harm in letting her walk the dog one evening since you typically allow her to do so even when the dog is at your house. I imagine your friend and your XW would be quite shocked if you made a stink out of it, they both probably see it as neither here nor there, right? So why did it upset you? Dig into that, there's something there that deserves exploration.
