Yes, in looking back, you are correct: The fault here is more with the wife. When she initially noticed the other guy watching, she convinced her husband that it would be exciting to let him keep him watching. She said nothing about allowing the other guy to physically interact with them. So in her giving permission to the other guy to touch and cum on her, she went beyond what was originally agreed to by her husband, which she should not have done. None of it would have happened (not even the continued watching) without her husband's permission, so anything more than that definitely should not have been done without her husband's permission as well.