Wife cheating on my brother in law

My brother in law got married one month ago. After their marriage she refused for a physical relationship as she told that she needs some time as she wanted to get mentally prepared. We all agreed. After a month her father and sister came to take her as she was going with them my brother in law just checked her WhatsApp and came to know about her affair which she continued even after her marriage. As soon as he came to know about the affair she deleted all the details from his phone and email Id. After talking to that my brother in law came to know that they were in a relationship and even had physical relationship. He talked to boy and he agreed about the relationship with his wife. His wife refused for the relationship with the boy after they three of them talked to each other for confirmation the boy to refused and said that they didn't had any relationship. All this started after my brother in law engagement. We just can't trust her she just flips time to time. After two days of marriage in the starting she demanded for divorce. Even she is a law student and she threaded us that she will commit suicide. Please suggest us how can we deal with this situation. As just we don't trust her we don't know she can do anything. Even her family was knowing about her affair and now they refused to take her garuntee. Please suggest us the solution so we can be out of this mess.