In the past ages ago the last time I had issues with stem rot the only thing that help was to clean the infected area with H2O2 (hydrogenperoxide) daily and try to keep a small fan blowing on it to keep it dry between applications.
I recall this is what I was told when I asked on OG atleast its what I ended up doing.
If all the tissue around the stem is gone you are going to have issues I think as that outer "bark" is what the plant needs to transport nutrients so if its completly gone around the hole stem only thing left is to hope it makes it but at the first sign that the leaves are drooping and not getting better cut it off just above the stem rot and put it in a bubbling bucket to re-root. This last thing also depends on how far along in flowering you are if you can even do it.
Good luck