Husband of girl asking to be witness against her else will file case

I was in a relationship with a woman who I later came to know was married.The husband got to know about us and collected proofs like call records and places we visited together. He was also involved in a relationship with someone else as what the lady tells me now and she have proofs like chat logs against her husband. The husband has approached me asking me to be his witness and accept that I was in a relationship with the woman so he can take divorce on grounds of adultery else he will file a case of adultery on me and I will have to fight a case as an accused. He assures that I wont be harmed in the process if I become his witness. Please help.