Many thanks for all the responses. I asked because I heard that if she has not made big mistakes, employer can't dismiss her, otherwise, she can sue the employer. Is that true? Also, I need to write a reason in the dismissal letter. But "unsatisfactory performance" is a good reason! Thanks.
Even you send the letter to Immigration, they also accept the reason of Unsatisfactory or your performance. So dont worry but I remind you that you should inform Immigration once you release her.