If its normal visit, what is there to blackmail? If there is nothing illegal/ unethical/immoral, how can she blackmail him. So there are other details in this story. I feel its good to inform someone in your family too. Else,he will put all his blame on you. But it can affect your married life. So do whats best for you. He shouts because he wants to silence you. You can talk to your husband and demand full details including her name, facebook page etc. Tell him,if he dont you will inform both sides of family as you dont see a future with these kind of lies. Stay strong.