Spraying the leaves now probably won't help since they are already burned. I would stop doing that since it would probably just encourage more fungal problems.
At this point, I would just improve the air ciculation but keep the plant out of direct sunlight. Here is is an under story plant anyway and does not like to be in more than morning sun. Watch the watering, don't over do it. If the stem remains firm the leaves may grow back. It may take a while since kava is a slow growing plant under the best of conditions. When we whack it back it takes a long time to grow back.