ANSWER:

If the fruits fall in their piece of land and not on his land, this is permissible in Sha Allah as he should have restricted the branches from trespassing. 

In My relative house,there is a guava tree which is owned by a non Muslim.My relative is eating the fruits of that tree without asking his permission.when I asked about that,they say that owner of the tree did not take care of it and it’s fruits become waste..so we are eating and we will give him money for that.Is it permissible? If not what is the expiation?