Help, Very old Xmas Cactus dying

[My PC]

Hi, I have a Xmas cactus that is at least 95 years old handed down
thru my family..I've had it 25 years...it was doing fairly well, just
bloomed right after Thanksgiving then  has begun withering slowly.
I thought it was dried out, then thought it had been overwatered...(my
wife is a covert overwaterer)....but on close inspection, the main
stalk? stem? is rotten.....its a large brown woody stem about 2 inches
in diameter....its now mush..there's still life in the long
branches..I've got to save at least some of this plant..What can I do.
Please help.