Walnuts fall on cars on the driveway, who is responsible for dent

[Marcin G.]

My rental has a driveway for two cars where tenants park their cars. They also can park on the street next to the house with 4 free township parking permits which they got. It happens that there is a large walnut tree close to the driveway and the nuts fall on the cars in the fall season. These walnuts are 3~4 inch in diameter and quite heavy, so they can make a dent in the cars I think: https://www.farmanddairy.com/wp-content/uploads/2014/09/DickieBirdFarm-BlackWalnuts.jpg 

The tenant wrote to me saying if I can trim the tree or find some other quick fix because they noticed dents on the hood of the car.

How do you suggest I should handle this request?