Thanks for the replies everyone.
Several of you said this is a "one time incident". Well, yes and no. The dog has been coming into our yard every single day for the last week, and doing its business. So my view is that it's really multiple occasions. Or at least, one incident stretched over a number of days.
I also find it hard to believe our neighbours aren't noticing the absence of doggie doo-doo. Their backyard is small and well maintained, so surely the absence would be apparent?
Anyway, the majority of you have advised us not to say anything, because of the favour Mr Neighbour is doing with the fence, so we won't press the issue.
I agree with your DH. I also think he can be the one to pick up the poop. I do think it's worth the "several hundred dollars" that you are saving on the fence.
DF gets very squicked out when it comes to chores like this. So guess who went out last night and picked up the 10 stinking piles? Yup. Me. DF wasn't going to do it. I used a shovel and wrapped it in layers and layers of newspaper and stuck it in our bin.
I really hope this darn fence gets finished soon!