Would this be a civil rights violation

Here is the scenario: (please disregard personally feelings about facebook,myspace etc.) (also this story is not about me however I do know the people involved)

Mr. X sends a message, while at home, and from his home computer, to Mr. Y via facebook personal message, the message was not threatening or any type of racist remark. Mr. Y is offended and tells his boss about what Mr. X said. Mr. Y's boss calls Mr. X's boss and demands that Mr. X write an apology to Mr. Y. MR. X's boss agrees and tells Mr. X that he will need to write an apology to Mr. Y immediately or face disciplinary action.

My question, is can Mr. X's boss threaten disciplinary actions for something that did not occur on work time, work grounds, or using work equipment? The only thing that is work related is that Mr. X and Mr. Y only know each due to work.

Also Mr. X went to his union and they advised him that he should just write the apology letter.

I am curios of your take on this, I feel that Mr. X has the right to express himself.