On the plus side...the house/property belong to Keith's parents so it's up to them, the parents, to decide what to do or they may decide to do nothing.
I like the idea of adding a bit of fence as suggested by
@dyzzypyxxy That would better define the space and may stop the neighbor's landscaper workers from trespassing.
How about a gradual, long term, pruning? Just snip one tiny branch this week...then one next week...little by little until gradually the shrub is within its own property?
