@santorini
I don't think it would be any different if they paid for half of the fence, the one putting up the fence is not going to put the 'pristine' side facing their neighbor's property and the 'ugly' side facing their own property..
We won't accept an offer of contribution when we make our fence change, even if it's offered.
If my neighbors don't like looking at the back of our fence, then they have the option of putting up their own at their own expense.
I'm very neighborly, but I won't let neighbors make decisions re: my fencing needs.
Luckily, they're the kind of neighbors who wouldn't expect to either..
You never know how strong you are until being strong is the only choice you have.