Getting things documented so that there is some disaster recovery capability is reasonable. Figure out that threshold, document that minimum, and then let the

Getting things documented so that there is some disaster recovery capability is reasonable. Figure out that threshold, document that minimum, and then let the person go. Possibly the situation changes while doing that, but if the decision to transition has been made, then stick to it. The severance package in one of the other comments is important. Even if the person has to go, the departure need not be as acrimonious.