If there was a witness to the assault the victim doesn't have to "press charges", if the witness is willing to swear out an affidavit the police can arrest him with or without the cooperation of the victim. Most if not all states have spousal abuse laws that take the burden of "pressing charges" out of the equation, the state prosecutes on behalf of the victim whether the victim wants them to or not.

Having said that, the karmic debt that this guy is building up is unbelievable, don't stoop to his level and seek revenge. I would make sure that the hospital/hospice is made aware of his actions as they can ban him from the premises regardless of his relationship to your SIL.

Everybody gets what's coming to them eventually...