Liz, if you followed many of my posts on this sort of tree planting and foresight you would see I'm as disappointed and concerned as you are.
Here's a thread on it.
Queensland Australia Fence Line Law
Whilst there's council regulations on even sheds people can plant whatever they like where ever they like. It is also done in an anti social behavioural sense where the trees are used as weapons in a fight to aggravate neighbours.
Then imagine and possible, down the track the tree could get protected taking right of way over what you do in your property. The tree has an allowance for roots to so if you wanted to re-landscape, excavate, put a pool in etc the tree owner could argue due to root damage what you do on your own land. In an extreme sense tree huggers or difficult neighbours could plant out their entire fence-lines with large trees and possibly control the neighbouring properties, totally unacceptable.
Simply unacceptable and clearly shows the flaws of landscape management and town planning.
If I were you I would approach the neighbour nicely and request it be moved, failing that then prepare written document perhaps by an arborist and "put them on notice". Send the letter certified mail and keep records.
Inform your local councillor of the inadequacies of the current state of affairs pertaining to tree selection and planting.
Fancy that, tree owners dictating what you can and cannot do on your property especially when coming from a trespassing point of view ... yes, both tree roots and canopy that come over your side of the fence are trespassing.