How can a person accusing the spouse of adultery prove it

My boyfriend wants to divorce his wife and marry me. But his wife said she will not give an easy one. She will file a case against him for adultery and keeping extramarital relationship with me. Will she have to prove this before the court? What can happen if she proves my boyfriend guilty? By the time we got closer to each other his wife also had sexual relations with another guy which she denies. My bf wants to get an easy and peaceful divorce by showing no fault against his wife but she is making all complicated. Can my bf get a contested no-fault divorce even if his wife tries to prove him guilty in front of the court?