You fired an employee yesterday for poor performance. Today you walk into work and that former employee is sitting at the bar. What do you do?

Ignore them and tell the rest of the staff to ignore them too. If they are dumb enough to come in and spend money, I'll happily take it.
Take the person aside, ask them how they are doing, and if they understand why they were let go. Give them an opportunity to reflect on what went down, and vent if they need to.
Go up to them and welcome them back as a valued customer. In fact, if it feels right, offer to buy them a beer.
Kick their ass out immediately I fired this person and don't want to see them again-ever.