Sometimes, line managers want an employee undergoing probation to leave by giving excuses like "he does not know his work". Bias and discrimination may be the real reasons rather than poor performance which is partly the fault of the recruiter for not hiring the right person and of the Supervisor for not training or teaching the new employee.
You should have already in your letter of appointment, a clause which says that during the probationary period of X week, either the employee or the company may give notice or pay salary in lieu. If you feel that the new employee is really a hopeless case and he has committed a serious infraction such as stealing, or hitting someone or sexual harassment, you need to be sure you have proof to terminate him without even giving notice or 1 week's salary in lieu.
I would check for REAL reasons why someone is asked to go during the probationary period. Most times, it is due to a mismatch. The Line Manager wants someone fast, who asks for low pay, has tons of relevant working experience, but has to be prepared to work for little. So, HR hires someone who may not be as experienced as needed and this is the result. In rare instances the Line Manager may fear that the new person is more competent than him and may take over his role in the future This happens if Line Managers are insecure and they give poor excuses why competent employees should be asked to go.