A "friend" of mine recently told my ex husband that when we were first separated, I tried to hire someone to kill him! What can I do legally? He doesn't believe it, but what if he has? And what if she told others this?

Someone has to believe it, the person saying it has to know it's not the truth, you have to be harmed financially.

Do you meet the criteria?

You can always retain an Attorney to write the person and threaten any and all possible legal action if his/her statements continue.

You are concerned about what your ex-husband thinks?

I know the ex doesn't believe it, but if she told him... who else did she tell?

Doesn't matter unless someone believed her. And if the police believed her you would have been arrested.

WAIT - who told you this? Your EX??
Before you get all up in arms, you need to confront your 'friend.' And if your friend denies it, then you need to confront your ex. And if your ex swears he wasn't just trying to rile you, then you need to drop it.

You need to be harmed by the untrue "information."

You cannot take legal action based on "who else" or "what if."

Again, an Attorney can write a cease and desist Order whether what you have heard is or is not true. If you then have a credible witness to the lies, then you can sue.