Please help me write a letter to the CEO about an employee grievance in a toy company.The company attorney notifies you that former employee has...

Your question is difficult to answer without more
information. For instance, who are you and what is your position or title in the
company? In what phase is the complaint, and what steps have been
taken?

It would seem that a formal complaint has already
been filed within the company and has come to the attention of the company’s attorney. 
I therefore assume the complaint has already been handled by human resources
(HR).

Assuming you are the employee’s manager, the usual
steps in handling the complaint you describe would be for you, the employee and the HR
director to meet with the goal of resolving the issue.  Before the meeting you would
have documented in writing for the HR department as much as you can determine about the
issue (including the information described in your question).  Failing resolution, the
company would have to make a decision as to whether or not to accommodate, in whole or
in part, that which the employee requests.  At that point the employee would be left to
agree or appeal.

It sounds as if the employee contacted an
attorney who notified the company of the intention to file a law suit, or possibly even
filed a law suit…hence, the involvement of the company attorney.  If this happened, then
you were probably asked to provide information as to the circumstances.  This may be the
letter you are seeking to write

If so, then the letter
should be written to the HR director, not the CEO.  And in any case, if a lawsuit is
involved, the information you write should be sent to a confidential legally privileged
file opened for this specific case.

In summary, you as a
manager are responsible for documenting in writing all that you know about the
grievance.  This information becomes part of the case file.  If a lawsuit is filed, the
company attorney and the HR department will handle it.  Your involvement would be
limited to providing the information as you already have, and you would not ordinarily
need to write a letter to the CEO.

The reference is an
excellent general discussion on handling employee complaints, ranging from specific
complaints such as you described, to serious company-wide issues such as a company’s
gender bias.