If you don't have an EAP, you can point the person to community resources for crisis intervention. I wouldn't try to counsel the employee yourself. It is not your role and will make it even more difficult to manage the situation down the road. If performance does not improve, then your responsibility is to take other corrective action, including reassignment, demotion, training, or termination, as appropriate. To do otherwise is to be an enabler to the abused person's self-destruction.
Some people have to suffer the most severe consequences of their actions in order to get help.
You aren't helping them by tolerating poor performance. It is very common for the victims of abuse to not want to press charges or see their abuser face any consequences for their behavior. I've had cases where I know that the abuser, if not removed by the court, will injure their partner again in the future. Believe me, no amount of confrontation or dire warnings by an untrained third party will make a difference when the codependent relationship is this destructive. If it is rumors, even if you saw the wife all black and blue in the grocery store yourself, stay out of it unless the abuser's performance or relations with coworkers is impacted.