It's libel. Plain and simple. She should press charges.
If she does, in fact, "somwhat abuse him," then it's not libel. Fathers get generally screwed by the courts in custody disputes, that's a fact. You should have seen the uproar when here in New Hampshire activists tried to pass a law to require the courts to mandate equal parenting time unless they could clearly demonstrate why they shouldn't.
You'd have thought they were demanding that the kids be fed into a chipper or something - the nerve to think that kids need their fathers!