[hooloovoo_zoo]

This is not a uniformly accepted rule. Commas are common after adverbs such as therefore, nevertheless, and hence. There is no mistake here.

[CalChris]

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.