Hi Ann:
I'm afraid reducing or trimming your neighbor's ginkgo tree from 40 feet in height to 8 feet in height (i.e., an 80% reduction) would be way too severe. At that point, complete removal would be a much better option.
Additionally, I'm assuming you're making this general inquiry on behalf of your neighbor...yes? I wouldn't want you to get in legal trouble for pruning or cutting a tree located on private property.