Yeah, that's what severance is for. Sign this paper that holds us harmless. We give you money to never see you again.

This is a problem that could be solved with money. If Charlie is as bad as described the boss would pay for them to go away instead of dragging it out through a drawn-out documentation process.

So, the problem is money just like the previous poster said. You can pay the poor employee for a few months while building documentation to CYA or you can fire them immediately and potentially owe them the 3 months (and likely more) from a settlement. Taking months to fire an underperformer is simply an exercise in risk management not a legal necessity.