Black mailing on basis of marriage certificate acquired fraduletl

A friend of mine is 25 yrs old. She was in relationship with a guy when she was 18. the guy influenced her and made her sign some document. to prove her love, she signed without questioning. Later it turns out, that the guy had got a marriage certificate issued in 2009 when she was 18. At that time, the only things that were checked for ceritificate were a sign of "pandit" , two friends, id proof which he got easily from her and the sign of girl and boy. Also, he submitted a photograph in which both were wearing "haar" which he got clicked in a studio as a part of some college function. The marriage certificate mentions that marriage was done under hindu customs. But they have not married in any temple or anything. there are no pics or video or any other proof of marriage like kanyadaan, wedding invitation, etc. The guy is harassing the girl a lot and pressurizing her parents. Is there a legal way to prove that there was no marriage done. or is divorce the only option? they have never stayed together all these years. Please suggest.