Since the fence is on the property line, you’re equal partners in its upkeep. This means you’d have to split the cost of repairs. But there are exceptions.
If your neighbour wants to rebuild a “fancier” fence than what’s necessary, then you would have to agree on how the costs should be split up. This fence is half yours, after all, and can’t be removed without you agreeing. And if you don’t agree, but your neighbour insists on the new fence, then your neighbour will have to pay for the whole new fence.
There could be other factors in play here. For example, if an upgraded fence is functionally necessary, which might be the case for a retaining wall, then you may have to split the costs.
To preserve a good relationship with your neighbour, you can consider contributing half of what it would have cost to fix the fence toward the new fence they want to build. In the future, you’ll probably have to split the repair costs evenly (if the fence is on the property line), so long as the repairs are reasonable.
You may find our page on fences and neighbours helpful. In particular, see the section on "work out problems."