Boundary trees are trees that cross property lines.
The location of the trunk generally determines ownership. If two neighbors are co-owners of a boundary-line tree, they must share tree-care decision-making, expenses and responsibilities equally.
Before taking any action, be it trimming branches or seeking legal action, have a face-to-face discussion with your neighbor and try to either compromise or reconcile the issue.
Encroaching Branches or Roots
Property owners in every state have the right to trim the branches or roots of a neighbor's tree that encroach onto their property, up to the property line, at their own expense. This right is called "self-help." However if you trim you cannot:
Harm the tree’s health, expose it to disease or destroy it
Trespass on another’s property to trim
Lean over the property line without permission
If the tree is on private property, any dispute that can’t be resolved by neighbors would be a civil matter. Consult an attorney.