Question
I'm located in Louisiana. My neighbor's pine tree limbs were over hanging over my main power feed to my house.  Numerous times limbs have fallen and knocked out my power.  The last time the energy company came out, they told me I was lucky because my main feed was almost ripped out my house.  I decided to remove some trees plus the limbs off my neighbors trees.  My neighbor gave me permission to cut back the limbs of her tree pine tree, but requested that the arborist apply black pruning sealer to the cuts.  When I had the work done, the neighbor was on vacation and the arborist cut the branches but said he wouldn't apply the black sealant because it is no longer common practice to apply the sealant because it actually can cause more damage than protect the tree.   The neighbor is still demanding that he apply the sealant.  By law, are we responsible for apply sealant to these trees where the branches were removed from her trees?

Answer
I will attempt to answer this from a scientific or aboricultural perspective, although you have asked for legal advise.

I am not able to provide legal council, however in that regard.
Did you imply or suggest in any way that sealant would be applied, or could be applied?  

Of course, even if you did, the judge would likely look at the arborist advise and current aboricultural or scientific best practices and lean toward those over any non-expert judgement.

In the past, it was considered a standard recommendation or practice to apply a generous coating of an approved tree wound dressing to all fresh pruning cuts.

This process was believed to prevent decay causing infection.  

This does seem to make perfect sense, at least from a human perspective.  We generally think about all wounds in the same manner as those of our own, that we are familiar with.

Remember a tree is not a human, nor is it even a mammal.  Although it is living, it does respond differently to injury and wounds than do we.  

In a trees natural environment branches will break from trees in wind storms, and the tree must have a way to defend itself without us applying a themselves without us applying pruning paint to every injury.  Now some breaks do cause serious injury and death in the forest, and to that we will also speak.

Remember and keep in mind that a tree does not heal its wounds or injury, but will close off or compartmentalize the area wounded.  This is referred to as "CODIT" and the tree does so by building walls around the area, of which we know there are four different wall types or locations.  Some trees can build these walls faster and in a more effective manner than others, and some walls in all trees are more easily established than are others.  

Others may think that pruning sealer will help prevent the tree from bleeding, but trees do not bleed.  They do not have any blood!!   The blood to you and I are crucial, as it carries oxygen to cells.  The fluid that comes from plants carries water, minerals, sugars, etc.  

Trees can loose a tremendous volume of sap without any major injury, it is only an irritant and not a critical issue.  The sap flow will stop naturally as the tree builds compartmentalization walls.

While Dr Alex Shigo debunked the myth of wound dressing decades ago, the issue still persists.  This is driven by our lack of education, old habits and vendors with products to sell.

Your arborist is correct in stating that pruning sealer or wound dressing is not generally necessary.  What is important is that proper pruning techniques are applied, thereby allowing the tree to close off the wound area and implement its own defense mechanism to the maximum benefit.

Scientific research has shown that any wound dressing or pruning sealer is not necessary.

Lets look further to substantiate this.

Edward F Gilman, in his book "An Illustrated Guide to Pruning", considered to be perhaps one of the best references on pruning practices today states on page 102.
"As a general rule, most pruning cuts do not benefit from an application of pruning paint.  Consult Camilli, Appel and Watson (2007) for a good review of current research on wound dressing applications on trees.  Several products are marketed as wound dressings.  Some may stimulate rot by trapping moisture, inhibit compartmentalization or serve as a food source for canker pathogens.  Certain dressings could injure tissue (Dujesiefken and Liese, 1991).  Oil based paint could be toxic to tissue bodies (Dujesiefken, 1999).  There is no scientific evidence that they help the tree close over the pruning wound, although lanoline may slightly enhance growth rate of callus over wounds the first year after pruning, especially when made in the dormant season (Crowdy, 2008).  If a dressing is to be applied for cosmetic reasons, apply only a thin coat"

Also reference ANSI A300 Part 1 - Tree Pruning.  The ANSI A300 is considered the standard for tree care in the U.S. and other parts of the world.  On page 24 it states "Wound dressings are treatments applied to pruning cuts or other tree wounds.  Traditionally, they are formulated with asphalt-based products in paint or spray form.  Wound dressings were thought to accelerate wound closure and reduce decay.  Research shows that these products do not reduce the spread of decay."  "Wound dressings are used primarily for cosmetic purposes, and neither are required nor recommended in most cases.  If a dressing must be applied, only a light coating of a nonphytotoxic material should be used".

In summary - it is generally of no benefit and not recommended.  

Where it is to be used, we generally will use a product called "Lac Balsam".  

It was developed for grafting and while there is not a lot of evidence it provides a benefit, I have not seen any negative issues related to the use of this product.  I am not in any way suggesting you use this product, but that it

http://www.greenlandgarden.com/talkshow/talkshow_tree_balsam.html

http://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=61&Type=2

http://www.treerx.com/product-category/lac-balsam-tree-wood-protectant/

http://www.treestuff.com/store/catalog.asp?item=1381

I would not recommend a black product, only because often times they are petroleum based.