But that's if you didn't make an election. Here, the participant did make an election - employer just didn't follow it. I'd say you refund it, plus earnings if there were any. Any match made, plus earnings if any, should be forfeited. I don't think that the employee failure to look at pay stubs is material here. I can easily see how this could happen - direct deposit of pay, one spouse keeps the checkbook up to date, other spouse doesn't even mention that elected no deferrals, etc., etc., etc.... - some people simply don't communicate.
Maybe that's a good thing - I've heard that communication is one of the main causes of divorce.