Try ti figure out what they have in mind and why that is what they are picturing. For example, someone will point to a neighbor's tree that has been maintained like a lollipop. I'll explain to them 2 things about that:
1) They've been doing that since it was planted...it is near impossible to expect an old tree to stay healthy if you cut it back that far and it will never be as full.
2) That is a very high maintenance practice. It is a good money maker for me, but I don't think it makes sense to do something that we know takes that much extra maintenance, is bad for the tree and looks terrible if you skip any of the maintenance.
Can I just help keep it away from walkways and buildings while maintaining the overall health of the tree? What would you think if we reduced all of the "outlier" branches and restored some of the trees's natural shape and symmetry?
And I always point out that with mature tree pruning "less is more". Every leaf is a food factory. An old tree is not as capable of rebounding from a very stressful loss - just like other organisms, including humans.