Is a contract legal when the girlfriend signed the contract

Hi I am purchusing a home on land contract and the guy i am buyin it from is trying to default on the home because i can not get insureance. Plus he has informed me that he has a buyer for the home for more money than what i am giving for the home. Then i recieved reciepts from his girlfriend and realized that the signature was hers. the bad part is that Wilma Van order the realestate agent notorized the document. the Contract was signed before we reached the office. Now would this make the contract null and void. I have made good on my part. the insurance problem is that he lied to me and told me that I could get insurance when the house is actually not insurable. It is crazy what he is doing to my family. I dont know what to do. What are my rights under a land contract.