Whether unregistered agreement valid in the court of law

Hello, I am asking this question on behalf of a relative. This relative of mine invested in a venture with a party without any legal documentation (i.e. on stamp paper), which I had advised against. Instead, my relative took the signature of the said party (cross signing on a revenue stamp) on a normal paper mentioning the details of the investment and returns with me countersigning as a witness. Now, my question is, in case of non-repayment by the party, will this paper be valid in a court of law? The location is Guwahati, Assam