If you find out your spouse is cheating on you, is it legal or are there legal ramifications for telling the spouse of the other party. For example

I believe your question is one regarding civil liability as opposed to a domestic matter.  You cannot be held liable for relaying information that is true.  If it is not true, or you do not take reasonable steps to ensure it is true prior to disseminating the information, you could be held liable for defamation.