Most likely, the tree won't die, at least not immediately (within 2 to 3 years.) Also, the face cut shouldn't be much more than 1/3 of the way into the tree (else the guy is a noob at felling trees).
Regardless though, even 1/3 of the way into an 18" dia tree is quite a bit of open space inside the trunk. And, that sort of wound to a tree is going to take quite a while to heal over (if ever), while still leaving a hollow space inside the tree. For safety's sake, I think I'd remove the tree & replace it. If the cut were only 1 or 2 inches deep, I think I'd hope for the best. But 1/3 of the way is too far (imho).
If you have wood of the same species and can cut off a couple of wedges that would be tight in the saw's kerf, it might help to pound them in, trying to line up the outer bark & layer just under the bark with that of the tree, and keeping it pretty tight when you insert it.
I think you would be surprised. If the cut is only the thickness of a saw blade(read: like a few mm), it might be able to seal itself within a few weeks if it's a fast growing variety.
I think the most important thing would be to dress it with some fungicide. Fungus or structural weakening will be what kills the tree, not the act of cutting half way through it.