You might be right. She may have used hence in the conjunctive adverb sense. Having looked it up a bit, it still goes both ways. The argument against the comma

You might be right. She may have used hence in the conjunctive adverb sense. Having looked it up a bit, it still goes both ways. The argument against the comma is that if hence is a transitional word then you would omit the comma. If you feel the need for a pause then you would insert it. So it's a fielder's choice and not a mistake.