I've been sitting here trying to think of the best way to respond to this. First off, I am sorry to hear about the situation you are in. That must be very difficult for you right now. Employers have different ways of handing situations like this. Should you decide to write this letter you would want to use a standard "business letter format". Here is a link to a layout of this format:
Try to obtain the name of the individual's direct supervisor so you may address it to him/her. This is not a letter that should go to a general mailbox with a "to whom it may concern" heading, as this is private information. If you don't know the name of the immediate supervisor you would want to contact the company's human resources department and ask what the best avenue would be to file a formal complaint against that individual. Remember that discretion is paramount in something like this for legal reasons.
In the body of the letter make sure to keep it simple, accurate and free of personal remarks. List factual information with a brief background to give whomever reads it a clear view of the situation. For example, "I am submitting this letter to inform you of a possible breach of your employment policy between one of your employees, Jane Doe, and my husband, a client of your company." Then you must include information that details how these actions violated company policy, for example, "I was made aware by my husband on Dec. 12, 2009 of an instance of infidelity in our marriage, caused by a relationship between him and your emploXXXXX, XXXXX." Keep the tone professional and the letter short as it will get the most attention that way.
Depending on the employer, this could be handled in a number of ways. It could be simply a confrontation between supervisor/subordinate or it could be reviewed on a larger scale by a panel or even an external panel, depending on policy. On a personal note, I would like to mention a few things though. You might want to consider if the woman in this situation did this to hurt you and your family or if she just had a terrible lapse in judgment. Remember that you are attempting to have her fired for this, and that is nothing to be scoffed at, especially in these troubling economic times.
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