I can't think of much you can do to save it, other than performing a bridge graft if it's large enough. Wrapping the trunk with wet moss would surely result in an air-layer if the top is still viable.

Trees are not capable of healing in the sense that new tissues grow to replace those lost at the wound-site. trees are regenerating organisms & you'll likely need to wait while the tree adapts to the injury, or replace the roommate - I mean tree. ;o)

Al

AND.....to prevent this from happening in the future, be sure to place a nice big mulch layer all up under the drip line of any tree or shrub that you might plant. Don't pile the mulch up on the trunk, but spread it out as far as you are willing to devote to tree/shrub roots. Keep the grass and weeds pulled within the mulch ring, and there will be no reason to mow or put a weed-eater up to the trunk.