Change locks, remove his/her alarm code. If they have remote access to POS or security cameras, change passwords. Explain to staff why the manager was terminated in general terms, no specifics and don’t try to make the manager look terrible in the staffs eyes. If manager is upset over being fired, keep your eye on social media and review sites for bad or overblown stories about your restaurant. To try to minimize the managers emotional response to the situation, be as professional as possible.