It is the Common Law thing. If it is not codified it makes it very difficult to set specific precedent, or cite one.
First rule of thumb is "Do No Harm." If you do prune back the overhanging limbs, do it right!
Second rule of thumb, don't cross the property line! So if you can't prune it right on your customer's side don't prune it!
That said, common law provides for your relief from encroachment, so you can prune the branches that poke you when you mow.
But if there are apples hanging from those branches...they're not yours, you cannot take them. That is until they fall to the ground, then they are your's and not your neighbor's. So, while they are hanging, you can't touch them and once they fall, you cannot make your neighbor clean them up.
So, now that we have cleared the muddy waters to a hazy soup, what are your client's rights?
Bottom line is that nobody here has taken an issue to court to get a legal precedent set. Without precedent, there is no clear direction to take.
Be friendly with the neighbors, get their permission, preferably signed.
All of our contracts have an indemnity clause where our client asserts ownership rights or having secured legal permission to work on all trees specified and that they will defend us and pay all legal costs arising from any disputes. While this may not protect us much, it makes our customers pause and get the permissions they need.
By the way, we always keep our eyes open for the potential boundary tree or encroaching tree. If we see one, we'll tell the customer we need their neighbor's consent before we begin.
Above all, don't upset the neighbor, thou you don't live there, your customer does, and you want to be invited back for good reasons, not bad.
Oh Yeah, in parts of California....If your neighbor's tree (on the other side of their lot) blocks your light...you might be able to exercise a legal right to force them to remove it.
So much for property rights!