Like I said, you can't do much about a stupid employer. (Doesn't matter where the supposed slander or "harassment" (if, again, one can logically call "get over it" harassment, occurred.) And a parent who isn't a conservator of the adult (let's say she's a disabled adult who needs a conservator) would have no standing to sue regardless.
"Forced by boss saying, sign this or you will be fired.."
That's still not a situation that calls for describing it as "forced"; whether the alternative was having a job or or not, your husband couldn't be "forced" to sign it. (There's also the matter of fact that the employer needn't have had him sign anything; it could simply say "don't talk to her ever again or you'll be fired.")