Review the item:
When Millie overheard Coach Jones complain that women did not belong on
a basketball court, she became so angry that she picked up a tennis racket
and whacked him over the head, the assault and battery
charge,
in Millie's opinion, was worth it.
The assault and battery charge was worth it is a main clause that can stand alone as a complete sentence. In Millie's opinion is an interrupter. Since it is breaking the flow of the sentence, you must use a comma both in front of the phrase and behind it.