Halle Berry and Eric Benet

I guess that just as the law forbids secret recording or photographing of "a third side" but permits recording or photographing acts in which the person recording or photographing is involved - it regards the act of gossiping against a "third side" as libel and an act in which "the informer" tells about himself or herself and of another person - as legal.

There is a civil claim might cover this situation:

"When can an individual sue for public disclosure of private facts?

Generally, the material published must be private information that 'is not of legitimate concern to the public.'  Its disclosure must also be 'highly offensive to a reasonable person.'  Material private enough to trigger this tort claim could include disclosure of sexual orientation, medical history, or other personal, private facets of a person's life.  The pressing question in public disclosure of private-facts cases is whether the information is newsworthy or of legitimate concern to the public.  ..."

(First Amendment Center)