special marriage act

one of my male friend who is hindu by religion and his family is also hindu is married to a girl(converted christain) but girl's family is hindu under special marriage act, as the community is different. The boy is a bengali community and the girl is from bihari community.The marriage ceremony took place as per hindu rituals.The girl hasnot disclose about her religion to her in laws who believes her to be hindu but her husband known the fact.Now my friend wants to do a second marriage so what are the grounds of divorce in this case. if it is not in mutual consent.