I would paint the trunk, wound and exposed to sun branches with IV organic tree paint to not let the bark get damaged further by the sun and insects.

I have no idea -- but my avocado tree (planted only a year ago) was looking super healthy and good, and within about 2 weeks all the leaves are black, the stem is black, and it looks dead. Almost looks like someone sprayed it to kill it. But I don't have a gardener, so don't think this is a possibility.

That does not look like root rot. Various types of root rot will show uniformly pale leaves that are undersized and wilt prematurely even in the slightest heat. The dead tips on the leaves look like excessive chloride in your water OR just very old leaves ready to drop. The lack of dark green in the leaves could be caused by iron chlorosis from high soil pH or lack of micronutrients. I would test soil pH, treat with iron chelate and a nitrogen fertilizer and also paint the trunk and other sun exposed branches as Sunshine said above.

did any of the suggestion work for you? My Avocado looks exactly like this!