A conversation is always nice and makes better neighbours, but I like the simpler route. As @jc100 mentioned they could still refuse, or agree and not offer half to pay etc.
The previous owners of where my idiot neighbours is, built the fence on their side didn't ask just did it. They actually did a bunch of cheap reno's to perk up the place just to sell. Now that fence in the back part of the lot is falling part at the bottom because they build it right on the ground, and didn't do the posts right. Anyhow if or when time comes to replace it I would refuse just spite this jacka$$ and make sure he has no access to my property to try and fix it.
Did I mention I hate that guy.
it doesnt matter if they refuse. they have to pay something