Firstly, ask her to not to disclose anything to anyone, and file for divorce by giving any excuse.
Secondly, do not thing of getting any alimony from the boy as later on he would prove very easily about her adultery.
Thirdly, he can file a criminal case against you if he happens to come or somehow bring any proof that his wife and you had intimacy after the marriage if any just on the ground that you know the fact of marriage.
Fourthly, and lastly, try to maintain distance till the time she gets divorce.