I nearly said something similar earlier, but I was worried about the libel laws!
He is now in the classic "disgruntled employee" situation, which was always near the top of the list of security risks that I would discuss with my clients. (Another one was over-reliance on one or more key members of staff - and we seem to have that situation as well.)
It beggars belief that he is still there, presumably with access to all systems and information. He should be out on gardening leave.
If he must be there to talk to external consultants or simply to bring the rest of the staff up to date, then all his computer system accounts should have been disabled, his phone taken off him, and he should be accompanied at all times. If the law allowed it, I'd strap his hands into boxing gloves, too, so he couldn't use any equipment