Is registering purchase of house at awas Vikas office necessary?

Hi sir, my grandfather brought a house from a lady. He paid all the amount. We are staying from the last 18 yrs in that house. That lady claim the case against my grandfather that he forcefully bought that house without pay the full amount. We have all the documents like registry, n all the signed notries which shows he made all the amount . But my grandfather have not register the name in awas Vikas office. What should we do. Is this serious matter. We received the notice from awas Vikas because that lady go to awas Vikas office n raise the request. Please suggest