How to file a case for property transferred by fake signatures

An old friend of our grandfather is claiming stake to our grandmother property in lieu for 30 lakhs paid for the property. however there was never any intent to sell the property, both parties never met,property sale was never discussed between both parties. but the grandfather's friend has forged grandmother's signature claiming money was paid for the sale but property was never transferred to them.witnesses are also not known to us. please do let me know how do we prove that no documents were signed and any kind of agreement was never made between both parties.