Note though, the above does not cater for Action Items which are active. These will have to be dealt with by the individual prior to them leaving, or, logging in with their account when they have left.
Alternatively, if the Action Item is coming from things like Risk/Issue Action Items, the PM can manually re-assign.
If the Action Item is coming from a Process, then consider modifying the Process to include a 'PMO' approval path, so that it allows the PMO group to approve the action item if required.
But in an ideal world, as part of the individuals exit process, all outstanding items should be identified and dealt with prior to them leaving.