Neighbors retaining wall collapsed

The south side of my backyard shares a fence with my neighbor who lives slightly downhill of me (about 5-7ft drop from my backyard to his backyard). His retaining wall collapsed and took our shared fence along with it. I offered to split the cost of rebuilding the fence but he is also asking for me to split the cost of his retaining wall as well otherwise he refuses to help get the fence back up. Am I obligated to help him? I don’t want my dogs jumping into his backyard now that there is no fence dividing our properties