Unless boundary fence owners agree otherwise, both owners are responsible for keeping the fence in good repair. However, the fence owners may agree to a different arrangement. For example, one neighbor might want to be responsible for repairing the fence, perhaps because she has a dog and is more interested in keeping the fence. Agreements between neighbors about boundary fences are only enforceable between current neighbors. When a new owner comes into the picture, the old agreement is no longer in force.
For legal advice, you'll need to talk to a lawyer.