If someone has “had the biscuit,” they’re definitely done, regardless of the circumstances.
The expression has its origins in a Protestant allusion to the Roman Catholic sacrament of Extreme Unction.
Biscuit is a contemptuous reference to the host, the sacramental wafer used by Catholics during the issuing of the last rites to a dying person.
If he’s “had the biscuit,” it’s all over.
And that’s why we say that someone who’s finished or fired has “had the biscuit”.