Made a purchase on gumtree yesterday, and scored this ficus (benji I think?) basically for free. Probably only cost me $1 with everything else I got with it

Now the base is showing a bit of rot, and I just want to make sure its not going to kill the tree. Is there anything I should be doing to prevent or stop this? Or can I just leave it as is?
I've put him under cover for winter to keep him from being constantly wet (in a little greenhouse I have) where he still gets plenty of light
Obviously there isn't a lot I can do now, but I just want to make sure it survives the winter
