I don’t think that’s an appropriate use of the word demise unless your manager friend killed the person in question. Which scummy move, for sure but I hope it didn’t actually lead to their demise.
Colloquially I think you may be right, but the dictionary definition makes it appropriate here:
Demise Definition & Meaning
noun ; a · death ; b · a cessation of existence or activity ; c · a loss of position or status.