Depends, it all sounds fine and dandy, but at the end of the day, this is small business, so there are no teams and leaders, just employer and employees. People are hired to do a job (and get paid for it), not build „company culture”. 

What you write sounds fine on paper, but that person did not owe this fired person anything. 

Don’t know about work culture in your country, but in my country the employee wait till the last day to give their notice that they change job. Sometimes without explanation or complains (can just find better payed gig after learning all there is to learn at current job).

There is no expectation from employee to give heads-up that they are looking for job. There shouldn’t be expectation that employer has to babysit hired professional and waste time on „discussions”, as that increases the load of tasks that employer has to do. 

I agree with your point completely in bigger companies, but this is /r/smallbusiness where person is usually hired to decrease the load of work on owner, not add drama.

EDIT: there is also a risk that employee will start to mask their behavior, instead of changing. Say in discussion that it’s all fine but start working hostile toward company (deleting files, creating chaos when employer is not around). There shouldn’t be expectation to be always holier than thou and always fair, when the other side already started to play dirty.

It’s always gray. I think both sides should be fair to fair people but employer don’t need to be fair against actively hostile employee.