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Legally neither neighbour can change or add to a boundary fence without the consent of the other party. It can only be maintained or, if it is destroyed, replaced like for like. Your remedy is to apply to the court for interdict which is an order preventing them from touching the fence. If they do so they will be in breach of the court order and could be fined or imprisoned for contempt of court. Happy to discuss. I hope that helps. Please leave a positive rating so that I am credited for my time.
If he has already started it then you apply to the court for a different order, called an order for specific implement, to reinstate the fence as it was.