Thanks for all the help.
What frink/Rathji said essentially mirrors what I thought before posting.
a) He has the right to trim up to the line (as his property extends from top of soil to space I believe), but if he kills my tree he is liable.
b) I have to ensure my tree isn't a danger.
The gentleman in question is a ~90 year old retired farmer who spends his days building various things out of 2x4s. That said, he sure as shoot won't be trimming any of the branches himself (see size below), my concern is if he gets Joe Blow Kijiji Inc. to come and they irreperably damage my tree. (Sure I could sue him, but odds of collecting (and doing so before....the inevitable) are low.)
His daughter runs a small dayhome out of the house and claims because of that, they arn't allowed to have branches over the property, which is why they want the branch trimmed. Given it sounds marginally like a legit excuse (never heard of such a thing before but I digress), is the reason why I was willing to trim the tree at all (and again, offered to pay for half).
Bonus Pic; as you can see its a huge tree (compare it to the scale of the standard size fence)
