Are there any conservation groups that can help to protect an old tree from being cut back?
I truly fear cutting the tree back will kill it.
You are being asked to prune an old tree that extends over your neighbors property and am seeking help in dealing with pruning an old tree.
In addition to concern over the health of the tree, there are legal issues in pruning the tree, i.e., your rights, the neighbors rights, etc. The best assistance that you can get would be from a certified arborist. You can locate one on the ISA web site; http://www.isa-arbor.com/findanarborist/. You can locate one by enterring your zip code.