Even assuming that Frozone's wife does not actually know that he has still been engaging in unsanctioned superhero activities (a highly dubious assumption), her relative willingness to let him go out as a costumed vigilante is not so unreasonable. People do illegal things all the time, and their families often take it in stride when they find out.*
This woman married Lucius Best knowing that he was a super who was devoted to helping people. She presumably admired that about him, and she loves him. Even though she apparently thinks that he is overstating the seriousness of the situation, she does respect his judgement about whether he can make a useful contribution.
*As Karen Hill states in Goodfellas, about a much nastier group of criminal husbands:
After awhile, it got to be all normal. None of it seemed like crime. It was more like Henry was enterprising, and that he and the guys were making a few bucks hustling, while all the other guys were sitting on their asses, waiting for handouts. Our husbands weren't brain surgeons, they were blue-collar guys. The only way they could make extra money, real extra money, was to go out and cut a few corners.